Isotopy and relative atomic mass calculations (full lecture)

Added: 2026-05-05

[00:00:04]All right guys, let's get into a new concept in chemistry.

[00:00:14]And what we'll be discussing in the course of this particular class lesson is this writing on the board. It is simply a very very vital aspect in chemistry called isotopic and relative atomic relative atomic mass calculations. S so please listen properly. All right. So first of all you can see what I'll be discussing in the course of this class lesson. This class will be divided into two parts. The first part of this class is for us to understand what the term isotopy means. While relative mass calculations. So listen properly. So first of all we have we have to know what isotopy is. Thank god I have done the introductory aspect to the atomic structure. So when I start saying things it won't be new to you.

[00:01:39]Now listen what is isotopy?

[00:01:42]Isotopy. Now first of all before we proceed the word ISO in science means the same.

[00:01:53]So the question you yourself what isotopy?

[00:01:59]Isotopy is a phenino.

[00:02:03]Please listen.

[00:02:06]Isotopy is what? A phenomenal whereby atoms of the same element relax and relax. Listen to what I just said. Is a phenomenon whereby atoms of the same elements have same atomic number but different mass number. Let me explain what I mean by what I just said. Isotopy is a phenomena whereby atoms of the same element. So relax. Let me put full representation on the board. So it tell me which is correct. Relax. Based on what I just said. See you just relax.

[00:02:55]Listen properly. This is oxygen 168. And this is another form to which oxygen can exist. And this oxygen now is oxygen 188.

[00:03:11]And we have another let's say nitrogen 147 and chlorine 3517.

[00:03:26]I believe this is not new to you seeing all of these things are not new to you anymore because we have used all of these to talk about how we can get the numbers of protons, electrons and neutrons in atoms. All right. So now looking at this what is isotopy again isotopy I told you is a phenomenon whereby atoms all of them are atoms of the same element. That is my definition. So please tell me which is correct. Is it this one or this one down? These two guys down or is it these two guys? Because you have to understand your definition. Isotopy is a phenomenon whereby atoms both of them are atoms.

[00:04:11]Both of them are atoms but of same elements.

[00:04:17]So for you to be called isotopes for you to be called what? Isotopes you must be made of the same elements.

[00:04:31]So tell tell me which is correct.

[00:04:35]Are these isotopes? They are not. Why?

[00:04:39]Though they are atoms, but they are not of the same elements. So that's why this one is wrong. So let's proceed. Isotope is a phenomena whereby atoms of same elements have same have same atomic number. Remember I taught you where atomic number is located. Atomic number is located down.

[00:05:02]have same atomic number but different mass number.

[00:05:10]So the mass numbers you can see that they are different. So this explains isotopy.

[00:05:17]I believe you are following me. I believe you understand properly what isotopy is all about. They are all very easy. So let's get into we are done with part part one. We are done with part one guys.

[00:05:32]We are done with part one. Part one doesn't have a lot a lot of story. So let's get straight into part two of this class which is relative atomic mass calculations. All right. We are interested in what we what we need. So you just listen.

[00:06:02]So moving further relative atomic mass calculations I'll call them RAM are you there relative atomic mass what calculations ram now to solve you know this is a calculative aspect now there there there is a formula you must note any day anywhere that formula you will use to solve reli* m1 which I'll tell you what it means divided by 100.

[00:06:57]All right, relax. Just follow me. Plus percentage 2* M2 divided by 100. This is the formula you must note first if you want to learn how to solve relative atomic mass calculations. Are you there?

[00:07:21]So moving further, what do we do next? Listen, we have to now know what each of the parameters in the formula means. I'm going to bring them one after the other. This is percentage one.

[00:07:41]The next, let me just bring percentage two so that we go together. Percentage 2. This is M1. I'll write it out. What does M1 mean? I will tell you very soon.

[00:07:50]And lastly, what does M2 means? I will tell you very soon as well. Now listen, percentage here, this symbol percent simply means what we call percentage abundance.

[00:08:10]All these questions on this topic are so easy. You don't have to stress yourself.

[00:08:16]Percentage here simply means what?

[00:08:17]percentage abundance. I just told you that percentage here means percentage of abundance. I did not tell you that percent I not I've not talked about the one. Do you understand? I've not talked about the one yet. Please follow me.

[00:08:34]Listen listen. I'll tell you what that one means. But let's talk about percentage. Next one. This other percentage you are saying it al it also means percentage of abundance.

[00:08:49]Are you there? Where m1 means m rather m itself is mass number. I have not talked about what the one means as well. Where this other m is also mass number. I have not talked about what the two means as well. Please anywhere you see one one simply means first isotope.

[00:09:12]Anywhere you see one it means what?

[00:09:16]first isotope I will tell you how to identify first isotope. So this now is percentage abundance of the first isotope which I will explain to you how to determine them. While for pre is two now means second isotope. So presented abundance of the second isol I believe you are following me. M1 means M one one is first as well. So mass number of the first isol to are you there? M2 means mass number of the second isoto. So as a student how do you determine first and second isotop?

[00:10:18]Let me draw isotop for us. 068 and N 147.

[00:10:29]Is this an isotope? So you want to write this in your notes. Don't do it. So it's wrong. They are not isotopes. Why they are not of same element? I'm going to wipe this. You understand? They are not of same. Don't do that. It must be of same element. We call them isotopes. So this they'll give me the question. So don't stress yourself. So this is oxygen 168 and oxygen 188. How do you identify which is first isotope? How do you identify which is second isotope? Now listen properly.

[00:11:03]It's very easy. Listen properly. Are you there?

[00:11:09]See, see when you are counting which do you count first? Is it 16 or 18?

[00:11:16]You count 16 before 18. So this tells us that oxygen 16 here is the first isotope of this oxygen we are looking at here whereby oxygen 18 here is the second isotope. Are you following me? Please listen. So in my question, anything they mention first, any value I see first should be for my first isotope irrespective of how the question is coined. You just listen and follow instructions. All right. So with all of this said this we've talked about the introductory aspect. Let us start solving question.

[00:12:05]All right let us start solving questions guys. Let us start solving questions based on what we have just said. Please pay attention. Question one. As I solve you will solve. As I solve you solve.

[00:12:21]Question one says, oxygen is a mixture of two isotopes.

[00:12:41]Oxygen is a mixture of two isotopes.

[00:12:44]What are they? Oxygen 168 and oxygen 188 with percentage abundance with percentage abundance of 90% and 10% respectively.

[00:13:15]ly listen properly. The question now says the relative the relative atomic mass of oxygen.

[00:13:37]Relative atomic mass of oxygen is a 16 B 16.2 2 C 17 D 18 Now let me explain guys listen very well there is something I have to tell you first of all before you even start solving questions on relative atomic mass calculations let me tell you something you must note this point you must note here that the relative atomic mass of most isotope of most elements in chemistry is not a whole number. This point must be noted. Please know this thing. Oh, it will help you a lot. I will explain what I mean by it is not a whole number. What are whole numbers? Whole numbers are not like 1 2 3 4. They are numbers without points. These are what we call whole numbers. So now that means our relative atomic mass is not a whole number. That means the relative atomic mass we have to sle should have a point. Whole numbers are numbers without point. But I'm telling you now that our relative atomic mass is not this ones without point. Sorry it's not it's not a whole number mean that it is not a number that has just one two three. You understand?

[00:15:06]That mean our relative atomic part should have point point.

[00:15:11]Do you understand? I'm just trying to use like man understand. So you get me understand I'm saying like our relative should have points in it. Please tell me the answer. Look at all of this option.

[00:15:21]I tell me the answer. Don't waste my time. The answer is option B 16.2 because this is a jump pass question because it was the only option with point so and that is what we want and that's how relative atomic mass of isotopes are generally in chemistry. any relative atomic mass you have to solve for your answer must be point do you understand but to be sure let us solve how does it work first of all you can see the various parameters let's bring them here the first one here is up percentage one the next one here is m1 the next one here is percentage 2 and the last here is m2 so listen properly what do we do they are all easy what is percentage one. Percentage one is the percentage abundance of what? The first isotope. This is my first isotope. So the percentage you saw first should ascribe to the first isotope. So which percentage did you see first? 90%. So percentage one is actually 90%. Are you there?

[00:16:31]What is m1? M1 is the mass number of the first isotope. M1 is what the mass number of the word first isotope. Now look at both of them. This is my first isotop 16. So my mass number is the one that is up. I told you guys already 16 in this case. What is the percentage two mass number percentage among others rather of the second isotope 10%.

[00:17:00]And what is the mass number of the second isotope? 18. All right. So with all of these let's sol for the relative atomic mass together. All right let's sol for the relative atomic mass together. Please listen carefully.

[00:17:20]So relative atomic mass ra will now be equal to what percentage which will have to be 90%.

[00:17:30]So 90 * N1 match number one 16 divided by 100 plus what percentage two which is what 10% times what mass number two which is what 18 / 100. When we press our calculator carefully we'll get 16.2 and just have like just 16.2 two as relative atomic mass of the isotope. All right. So you can see how easy it is. So get ready for your own practice question. We are going to solve all questions on this aspect. Get ready for your own practice question. Can I wipe the board now? Can I wipe the board now?

[00:18:19]Yeah, I can. So get ready for your own practice question. They are all very easy. I'm about wiping. So we can get into more approach on this aspect. So this is your own practice question.

[00:18:35]This is your own practice question guys.

[00:18:39]This is your own practice question.

[00:18:45]So this is your own practice question based on what I just explained. It is very very easy like ABC. Your own says calculate calculate the relative atomic mass ra of chlorine of chlorine of chlorine. You can see that it is not always oxygen.

[00:19:11]They can give you of different elements of chlorine which contains all right which contains which contains 75% chlorine 3517 and 25% chlorine 3717.

[00:19:40]will stop.

[00:19:42]Option A 3 5.6 option B 35.5 option C 36.5 option D 35.25 25. All right, you can see how this question is there. You can see all of them has point. So you must be careful.

[00:20:11]All right, you must be careful guys. So let us proceed. Now let me tell you something. If you observe, you can see that we always use like two isotopes. We can also see questions where they give us three different isotopes, three different percentages. What you have to do is still the same approach. Add to your formula. So you have to add to what your formula. For instance, it's not deal with three set of isotopes, three set of percentages, three set of mass numbers. Add to your formula. Do you understand? So 3 * m3. Do you understand? Divided by what? Add your formula. Don't stress yourself. Don't stress yourself. Let's get into more ways questions can be asked on this aspect. All right. So let us move further. Let's get into the question the ones that it deals with relative atomic mass calculations dealing with ratio.

[00:21:14]So in those kind of questions you'll be seeing ratio ratio ratio ratio. What will you do? So listen and pay attention. Can I wipe? Yeah, I can. So now this time around this deals with relative mass calculations dealing with ratios.

[00:21:33]Dealing with ratios. What do we do? What do we do? Let me tell you what to do on relative mass calculations dealing with ratio. This is what you do.

[00:21:48]So relative mass calculations ratio that's RAM is this the formula whenever you just see ratio in the question that formula that's percentage you you remove it from your mind I use this one I'm saying it is simply ratio one please listen ratio one time M1 doesn't change divided normally it was percentage you by 100 but because it is ratio we divide by ratio total please we divide by what ratio total plus ratio 2 all right plus ratio 2 * m2 / what ratio total this is the question this is the aspect we want to start talking about all right so listen and pay close attention So here is your own not your own like my own and after I'm done solving I'll give you your own. So now let me write the question on the board. It says an atom please an atom has two isotopes. They told us that it has two isotopes. All right two I just three one two and three. You add ratio 3 * mass / 3 over ratio to right. So ask two isotopes. What are these isotopes? X 20 10. All right. And X 22 10 in ratio in the ratio 1 ratio 3 respectively.

[00:23:41]All right, question now says calculate the relative atomic mass of X. So you're looking for the relative atomic mass of this element which is X. Do you understand? So what will you do? The same approach it doesn't change. It doesn't change. The same approach you simply just come here and say relative atomic mass is equal to now ratio one there's no need to start writing list parameters ratio one is the first ratio you saw in the question between these two ratio which is the first one you saw see it is not because I'm having one that's why ratio one was one is a coincidence this ratio can be any value can be 1 2 3 1 million do you understand So first ratio I saw was 1 * m1 mass number of the first the first isotope now 20k 22. So the first isotope which is 20. All right. So here become 20. All right. Divided by ratio total.

[00:24:49]To get ratio total very easy. Ratio total will now be addition of both ratio 1 + three. These are the two ratios. You add them together and that is four. So ratio two that becomes four plus ratio two. Ratio two is the three. All right.

[00:25:07]The second ratio. So 3 * mass number of the second isotop 22. All right 22 divided by ratio to as well four. So what becomes the relative atomic mass when we hit our calculator. What becomes the relative atomic mass when we hit our calculator? It's going to be 1 * 20. All right divided by 4 that's 5. 3 * 22 / 4 that's 1.

[00:25:35]3 * 22 / 4 that's 16.5. So 16.5 + 5 that's 21.5. So everything becomes 21.5.

[00:25:47]So this is the relative atomic mass for this isotope dealing with ratio. So I'm about giving you your own practice question.

[00:25:57]Let me see what you can do. They are all very very easy. You just listen get ready. So I'm about writing out your own now. So I'm going to change the isotope.

[00:26:08]So an atom X an atom has two isotopes which are 39 39 and 18.

[00:26:18]All right. And here is 40 and then 18 as well.

[00:26:25]You just listen in the ratio 3 ratio 2 respectively calculate the value of x the same thing. So you solve this is your own practice question. Solve and provide this in the comment section of my DM. All right. So that is for that. I have another one for you on ratio calculation related mass calculation delivery ratio. It says lithium exist as L I 63 and L I all right 73 in the ratio in the ratio to ratio 25 calculate Calculate the relative atomic mass of lithium.

[00:27:34]Please listen. Oh, calculate what the relative atomic mass of lithium. You can see same the same way. All right. So you can see that first we have talked about how to solve questions or relative atomic mass calculations dealing with percentages. And now we have learned how to solve relative atomic mass calculations dealing with ratios. All right. So let's get into more ways questions can be asked. Now this is the another set of question I want us to solve and we have to take our time to tackle this set of question. Now this time around then you will be given the relative atomic mass. What they will not give you is the percentage abundances of the two isotope of both isotopes.

[00:28:18]Relax, relax. And I want to write that question on the board because Jam W do bring that kind of question. Please note it well. I mean by writing it on the board first. Let me wipe and then let me write it step by step and explain to you step by step as well. And at the end of explaining, I'll give you all practice question. Please pay attention.

[00:28:41]Pay close attention.

[00:28:44]All right. Pay close attention.

[00:28:50]Pay close attention.

[00:28:53]So what do we do? Listen. First of all, let me write this question on the board so you see how it works. So the question says chlorine has two isotopes.

[00:29:10]Chlorine has two isotopes. What are these isotopes?

[00:29:14]CL 351 17 and um C L 37 17 if the relative atomic mass you can see they g they've given this to us already of chlorine if the relative atomic mass of chlorine is 35.5.

[00:29:48]The question now says what is the percentage abundance of both isotopes.

[00:30:08]Please listen and listen. Good. Listen and listen. Well, I repeat, listen and listen. Well, this question is very easy but sometimes smart or smart. It's not really hard though. Just listen. I beg you listen.

[00:30:32]If you observe this question first of all what are you even seeing because so far we have talked about two concepts on relative atomic mass. The first was solving questions on relative atomic mass dealing with percentages and the other was solving questions on relative atomic mass dealing with ratio. Dealing with what?

[00:31:00]ratio.

[00:31:03]So guys, what do we do?

[00:31:08]So what do we do now?

[00:31:10]You look at your question. You now look at this question. Is this question dealing with percentage percentage or is this question dealing with ratio?

[00:31:26]Look at the question from the beginning to the end. Did you see ratio at any point or did you see percentage at any point?

[00:31:33]Please tell me. I saw percentage. I don't know what you saw. I saw percentage at a point in this question. Relax. What they are asking me to calculate? I will calculate it. But first thing I saw percentage.

[00:31:49]Missing percentage. I am going to use the formula for relative atomic mass calculations that deals with percentages. So first of all my formula will be relative atomic mass calculations which is RAM is equal to P percentage 1 * what M1 all right divided by what 100 plus what P percentage 2 * what M_sub_2 / what 100 please listen oh because I did not see ratio I don't have any business with ratio here. Now let us read our question properly. I know what exactly they want us to do. Please listen. The questioner told us that this chlorine has two isotopes which you can see here. Right?

[00:32:42]They said if the relative atomic mass of chlorine that means in this question we are not we are not to calculate relative atomic mass anymore. They have given it to us and they said it is 35.5.

[00:32:55]We are not solving for it again. They've given us already. We are not solving for it again. They've given us already. I will note it. So I think what they want me to calculate for is what is the percentage abundance of both of this isotope.

[00:33:12]Please listen. You know normally this is my percentage one. I'll write it down.

[00:33:16]I'll say percentage one is equal to normally if it was those initial question they will give it to us. But in this case we were not given. I will make it X because this question said what is the percentage abundance of both of them. So you're looking for the two percentage percentage one and percentage two. Are you there? See? So next M1 is the mass number of the first. This is the first. What's the mass number? 35.

[00:33:47]Write it down. What about percentage two? Were we giving? No, we were not.

[00:33:53]What will Lesie call it? It is unknown.

[00:33:56]So I'll call it X. We are looking for the X. Now what is N2? Which is the other part of my formula. All right.

[00:34:04]Mass number of the second is in this case it is 35 37 rather. Guys listen listen.

[00:34:15]There is something I have to tell you.

[00:34:18]See listen as I tell you. Note it.

[00:34:23]Anything I want to tell you now, just put inside your head. Note it. Note it.

[00:34:28]See, I am aware they are asking me to calculate for percentage one and percentage two. I am aware they are asking me to determine percentage one and percentage two. In this question, which is percentage abundance of the first isotope and second isotope in this question. I am aware that is why I made both of them X. But please this is why I am teaching you please I am aware that percentage one is X. I am also aware that percentage 2 is x. But if we are about imputing all of this into the formula, please my percentage 2. I am aware it is x. But please note this thing if percentage where see where wherever we see percentage two when we are when we want to start solving wherever we see it please make it 100 - x don't just use x like that see note it wherever you see patent is two see don't ask questions. Yes, just relax. Listen to me. Wherever you see percentage two, don't write I I'm aware it is unknown. That's why I made it X.

[00:35:50]But if you want to start solving percentage 2, wherever you see it, make it 100 minus X. See, at the end of solving this thing, this question, you see the reason I said this. Did I say it is percentage one, you make 100 - X? Did I tell you that? No. I told you percentage 2 make it 100 minus x see we are okay let us start in all of these parameters into the formula so ram first of all let's look carefully 35.5 is equal to percentage 1 what is percent one that alter no title percentage one is still unknown x times what is m1 mass number of the first isotope. What is the mass number of the first isotope? What is it? 35.

[00:36:48]See, listen.

[00:36:51]Divided by words 100.

[00:36:55]Are you there?

[00:36:57]Plus percentage two. Now listen. What did I tell you? Did I tell you to just put percentage two as x? Don't do that.

[00:37:06]Don't try it. What? Wherever you see percentage, write 100 minus what? X. And don't just stress it.

[00:37:16]Put this inside a bracket. So this I just wrote 100. Just do it. Listen.

[00:37:26]Times M2. What is M_sub_2? Mass number of the second isotope. What is my second isotope? that chlorine 37 and what's the mass number? 37 divided by what? 100. So let us evaluate step by step. First thing first we will write 35.5 is equal to 35 * x that is 35xided by what? 100 plus what then happens here is that this 37 times everything inside the bracket. So 37 * 100 this will give us 3,700.

[00:38:11]All right. 37. No, this bracket. Any bracket is fine. 37 * 100 first. 37 * - x. 37 with time 100. 37 * what? - x. So, let's do that. 37 * - x - 37 x. See, do you know this thing? This is small math.

[00:38:31]Now, don't for my handle. So, then we say divided by what? 100. See this question I am solving. And I'm going to give you an exact same question, but this time around it will deal with another element. So please pay attention. All right, pay attention. Pay attention.

[00:38:54]Pay attention. Next thing you do is this. All right. Next thing you do is what? This. After you arrive at this point, you will then collect L terms.

[00:39:07]What this? No, no, no. You then do your LCM.

[00:39:11]LCM like if you observe you see that both of them have the same denominator, the same values that see whenever you have like a fraction. Both of them are fractions having the same denominator.

[00:39:21]Pick one. Pick one big 100. Pick one of them to come big 100. So it's going to be 75. Follow instruction. So whether I give you your own this what you do something else. So all right, follow instruction.

[00:39:36]Pick one of the big 100 and then add them together.

[00:39:43]Pick it. So it's going to give us 35x listen.

[00:39:48]Add pick one of pick join them together.

[00:39:50]Pick one of big 100. So it's going to be 35x plus 3700.

[00:39:55]All right. Minus 37x. Are you there? So pick one of the big 100. So I picked just one of the 100. Are you there? So next thing so then next thing we know cross multiply cross multiply cross multiply means that 35.5 * 100 and everything here 1. So let's do that. So it's going to give us 35.5 * 100.

[00:40:21]All right this is equal to this thing * 1. So 35x + 3700.

[00:40:30]All right. - 37x * Are you there? So let's time. So 1 times we're going to get 3 55.

[00:40:39]All right. That's 100 is equal to everything. All right. 35x + 3700 - 37x * 1 is still the same thing. Do you do you agree with me? Yes. So guys, what do we do next? What do we do next?

[00:40:57]We then collect like terms. What you think resemble? We bring them to one side. What do you think? See, if you observe 35 and 37, they look alike.

[00:41:07]They're like numbers without x 35x and - 37x they look like they should stay one place. So that's collecting like terms.

[00:41:14]Those that are you know looking like goes to one side. So see this let me tell you this plus 3 700 will push you to that side.

[00:41:23]It will cross equality and whenever a number crosses equality change signs sign. So here become 3 5 0 - 3700 is equal to 35x - 37x please just follow instruction. So this minus this we're going to get - 150.

[00:41:52]All right. Is equal to 35. First of all, remove that x. Just say 35 - 37. What will you get? That's -2.

[00:42:02]Do you understand? So then you're going to have - 2. But see there's x - 2x. Do you understand? So next thing we cancel - sign. So canceling minus sign we have something like this. So just 150. The reason why cancel minus sign minus will present in both side. So minus 150 fresh 150 without minus is going to is equal to fresh two without x. So 2x now. So then we divide both side by the coefficient of x. Meaning what is close to x that's two to get the value of x.

[00:42:37]All right. So then two so x now what?

[00:42:41]150 by 2 and that's 75. And it should be form of percentage. Do you understand?

[00:42:47]See listen. So okay 75. And now which of them is x initially? Well see see I know both of them. But which one did I tell you is x? Actually percentage one is X.

[00:42:58]So percentage one should be what X and what is the X? 75%.

[00:43:04]But how do we now get percentage 2? I've told you guys already percentage 2 be solved by saying 100 - X. So percentage now be solved by saying 100 minus the X we put in when 75. So 25.

[00:43:21]Now if you observe I think you solve the question that initially that we had you know the percentage abundance to be 75 and 25 in the question and you can see how we have cutting it back. You can see how easy these talks are. They are not difficult at all. All right they are not difficult at all. Are you there? So note all of these things you can see the two answer to the question.

[00:43:49]You can see how easy it is. All right.

[00:43:51]So note them guys. Note them. You can see how easy they are. Right. You can see how we solve them. That's why initially to get your percentage 2, it will not be worth 100 - x. Please don't forget. See 100 minus what? X. 100 - x.

[00:44:09]That's why 100 - x 75. And then that's 25%. All right. Is for that guys. That is for that. Let me give us another one.

[00:44:19]Master this. Look at it. Well, I'm about giving you your own practice question.

[00:44:23]Let me see what you will be able to work out. They all easy. Let me give you your own guys. I'm about wiping the boards where I give you your own practice question based on this approach.

[00:44:37]Can I wipe? Yes, I can.

[00:44:41]If you like pause the video and watch it, you see you'll be able to get it very well. All right.

[00:44:49]So as they are all easier they are all easy. This is your own guys. It says the relative the relative atomic mass the relative atomic mass of antimony antimony is 121.8 Eight. Stop. We now said that anti exist as two istopes which include listen antimony symbol is SB 121 and SB 123.

[00:45:51]Question now says calculate the percentage.

[00:46:00]Calculate the percentage of bondance of both isotopes.

[00:46:12]That is the question. It is similar to the one I just solved for you guys. So work it out.

[00:46:20]Work it out guys. Work this one out.

[00:46:23]They are all very very easy. Work it out. Work it out. So can go back to other ways question can can come on this aspect. Other ways questions can come on this aspect. So let's see how it works.

[00:46:37]I want to give us another entirely different question. And after this one is exactly the one I just solved a sample on the one I just solved. For this one I'm going writing is different and it says two isotopes two isotopes X A 29 and X 6529 with 80% and 20%age are born percentage change our bonds respectively.

[00:47:24]Wait with relative atomic mass of 64.2 Two question says what is a? See this one is so simple. See they asking for a what will you do? That mean asking for this guy and what this guy normally this guy normally is not M1 always. It's not C now it's not M1 always. Let's write our formula and get together. So relative atomic mass will now be worth one time what? N1. All right divided by what? 100 plus what percentage 2 * m2 / what 100.

[00:48:07]So since this is m1 based on what we are saying I can just remove m1 and put a instead I use the what they ask you to get. Do you understand? So what mass in the question 64.2 is equal to percentage one. This is the first percent you saw.

[00:48:25]So it's 80%. All right. 80 * a divided by 100. All right. Plus now listen percentage 2 is 20%. All right that's 20 * mass number 2 65 all right 65 divided by 100 hope you are following me now we've done this many times so what next we do so 64.2 2 is equal to 80 / 100 because 80 * 8 is 80 A + 20 * 65 / 100 20 * 65 / 100 20 * 65 20 * 65 / 100 that is 13. All right listen so then I'll come here and finish it.

[00:49:10]Listen listen listen I'll come and say 64.2 2 is equal to 80. Now listen, you can evaluate this 80 / 100 first of all you have to solve that 80 / 100 what's that is 0.8 just right here back oh 80 100 is 0.8 A2 and what is remaining a so you write it like that that's in mathematics it works like that when a number has something up unknown up you can divide you can divide them and just remaining a there do you understand plus what 13 please all these things are easy they're not hard so what do we do next we then you know collect like terms like this guy will cross so be 64.2 2 this + 13 cross equality going to be -3 is equal to 0.188. Are you there? Please do you understand? It's simple now. So 64.2 - 13 that's 51.2.

[00:50:06]So here becomes 51.2 is equal to 0.8. So divide both sides by the coefficient of words a. What is close to a that's coefficient of something 0.8 0.8 8. So 0.8 cancel 0.8.

[00:50:23]So what become? A 51.2 / 0.8.2 / 0 51.2 / 0.8 that is 64. Wow. We've got a to be 64.

[00:50:38]You can see how so this a m1 will now be 64. You can see how questions on this aspect are being tackled without stress guys. Actually um these are how questions smart I think I should give you some practice question. Let me see what you can do.

[00:50:58]Let me see what you can do. They are all easy. They are all easy guys. They are all easy.

[00:51:06]All right. So this is one question. All right. All right. Okay. You guys should just take these ones. All right. Um don't worry. Don't worry. I think with all of said we are done with this topic.

[00:51:19]Isotopy and relative atomic mass C there's no way your exam jam or bring questions on this that you can answer if you can work through remember all what we have done you are good to go so we are done guys see you when I see you God bless you all I wish you good luck in your exams