2022-2025 GCE Science Mole Concept Solved lPercentage yieldl l Percentage purity l Massl Calculation

[00:00:00]All right. Good afternoon everyone.

[00:00:03]>> [snorts] >> I welcome you to this wonderful platform. Of course, this is the mentor Humphrey Chimanga.

[00:00:09]I will be teaching you how to solve mole concept. We are going to solve particularly about uh three questions. And then the fourth question, you'll be able to solve. These three questions were actually taken from the previous uh years the previous past papers of course. So, this is actually a revision of all the the past papers of course the the science uh sections on mole concept so that you will be able to grasp the concept properly.

[00:00:42]Now, let's get started. Please, if you are new to the channel, you know what to do. Consider subscribing, hitting the like button, and also don't forget to share so that you can have access to all the uh content that I'm going to be making for you as we simplify your work.

[00:00:59]Now, [snorts] look at this. They're saying 4 g of impure iron oxide reacted with 600 uh cubic centimeters of 0.1 molar of uh per cubic decimeter.

[00:01:14]Okay, of sulfuric acid. Then they say construct the balanced chemical equation for the reaction. Then they say state symbols are not required. Now, I want to simplify this particular work so that it becomes very much easy for you.

[00:01:32]The first thing to do is simply to understand what is happening.

[00:01:36]They're saying you have iron three oxide reacting with sulfuric acid, right? So, first of all, how is the chemical formula of uh iron three oxide? Okay? Or let me just say the molecular formula. How is the molecular formula of iron three oxide going to be? So, this is how it's going to look like.

[00:01:59]We know that iron three is going to look like this.

[00:02:04]>> [snorts] >> Then we also know that an oxide is also going to look like that.

[00:02:10]So, the two are going to exchange positions.

[00:02:13]Then the three is going to come this side, the two is going to come this side. So, this is how your iron three oxide is going to look like. So, the two will come here.

[00:02:22]And then of course, you are going to have your oxygen there we say three.

[00:02:26]Okay? Now, it is this iron three oxide which is going to be reacting with the sulfuric acid.

[00:02:34]Now, sulfuric acid of course looks like this.

[00:02:38]Okay? This is going to be your sulfuric acid.

[00:02:41]So, let's try to react them.

[00:02:44]Of course, we are going to have our iron three oxide. This is going to be iron three oxide.

[00:02:51]Reacting with sulfuric acid.

[00:02:55]Okay?

[00:02:57]What exactly is going to be produced?

[00:03:00]Now, because you are reacting a metal of course with an acid, what you're going to produce is a salt which is which is going to be called uh uh iron sulfate plus water.

[00:03:17]Okay? So, now how are we going to write iron sulfate?

[00:03:20]Simple.

[00:03:22]Okay? This is how your iron is looking like.

[00:03:25]Because this is iron three.

[00:03:27]Then of course, a sulfate ion looks like this.

[00:03:31]So, this is going to come this side.

[00:03:34]This is going to come this side.

[00:03:36]Then you're going to have iron.

[00:03:39]Okay? The two is going to be here.

[00:03:41]Then of course, you're going to open the brackets.

[00:03:44]Then your sulfate is going to be inside.

[00:03:47]Then this three is going to come here.

[00:03:49]So, basically you're going to remain with iron two open bracket, then of course you're going to have your sulfate.

[00:03:57]Okay, on this particular end. So, this is what you're going to produce. You're going to have your iron two, open bracket, then a sulfate.

[00:04:11]Plus, of course, you are going to produce water.

[00:04:16]H2O. Now, they're saying we balance this particular equation. Notice that we have two hydrogen. Okay, let me just say this is my left. This is going to be my right.

[00:04:27]Let's start with iron. Iron is already balanced. We go to oxygen. Oxygen, we have this uh three oxygen here.

[00:04:36]Then we also have uh four oxygen. So, this is particularly uh 3 + 4 is like a seven, of course.

[00:04:44]But, this side, how many do we have?

[00:04:47]This side we have This is 12.

[00:04:50]This is 12, in particular.

[00:04:53]Plus this one, it's going to be 13. We don't want that. So, to balance this this particular equation is actually very simple and straightforward.

[00:05:06]Okay. So, how are you going to balance it?

[00:05:09]>> [clears throat] >> We we can put a three here so that we can have 15 this side.

[00:05:17]Okay, 15 this particular end. Then, let me see.

[00:05:23]We can also put a three here.

[00:05:26]So, if we put a three there, let's check. Iron balanced.

[00:05:29]Okay.

[00:05:31]Let me check what we're going to have.

[00:05:33]Of course, sorry, this is supposed to be iron.

[00:05:35]Supposed to be a two here, because this is what we wrote here.

[00:05:39]Okay, let's check. Iron balanced.

[00:05:40]Oxygen, how many do we have?

[00:05:42]Oxygen this side, we have this three is multiplying this four. So, this is 12 + 3 + this [clears throat] three here.

[00:05:49]This is going to be like 15. This side how many do we have? This four multiplied by this three, 12 + this three, 15. Okay, so everything is actually balanced. Now, we have we have a balanced equation.

[00:06:03]The question is now saying we calculate the mass >> [clears throat] >> Sorry.

[00:06:09]The mass of the pure oxide that reacted with the acid.

[00:06:14]Now, this is how you're going to calculate it.

[00:06:18]Let me just uh clear my board.

[00:06:24]This is how you're going to calculate it. Check what you have been given. You have been given the volume. You have also been given the concentration. So, for sulfuric acid what you have been given is you have been given the volume which is 600 uh cubic s- uh of course centimeter.

[00:06:44]But, you're supposed to convert it into deciliters.

[00:06:49]Okay? So, you just divide by 1,000.

[00:06:53]And then you say 600 / 1,000 you're going to get 0.6.

[00:07:02]Okay?

[00:07:04]Cubic decimeters. Okay?

[00:07:09]You have that.

[00:07:11]Those are the correct units because that's what they are using here.

[00:07:14]Then, what do you have as a concentration? Your concentration or molar Let me just put C for the sake of structure. So, this is going to be C uh representing concentration which is 0.1 0 0 then mole per cubic deciliter.

[00:07:39]Now you will to calculate your number of moles, which we don't know.

[00:07:45]So, we know that concentration is equal to the number of moles over the volume. So, if we cross multiply, number of moles is equal to concentration multiplied by the volume.

[00:07:56]So, number of moles is equal to What is the concentration? Concentration is 0.1 multiplied by the volume. Of course, the volume is here.

[00:08:06]So, our volume is 0.6.

[00:08:08]So, the number of moles is going to be 0.06 moles.

[00:08:15]These are the moles of what? They are the moles of sulfuric acid.

[00:08:22]Now, what you want is the mass of the pure oxide. So, what do you do?

[00:08:26]You use the balanced chemical equation.

[00:08:29]Please, when you get in the exam, when they ask a question under mole concept, make sure that you have your balanced equation, always.

[00:08:38]So, now, let's get our oxide. Of course, our oxide is here. Our iron three oxide.

[00:08:46]We are going to use the equation. We are comparing it with what we already have, which is our sulfuric acid.

[00:08:55]Then, of course, use the balanced equation because here you have one mole, there you have three moles. So, you say one mole, this side you say three moles.

[00:09:06]This side is what you want. This side you already have the moles.

[00:09:11]So, you put your 0.

[00:09:13]06 moles.

[00:09:15]You cross multiply. Your X is going to equal to So, this is going to be 3X then 0.06.

[00:09:24]You divide this side.

[00:09:28]So, 0.06 / uh 3 and then you're going to get a 0.02 moles. Of what?

[00:09:38]Of iron two or iron three iron three oxide. Now, we have the number of moles, everything becomes easy now. Because we already know that number of moles is equal to mass over the mister.

[00:09:55]So, the number of moles we have what we want is the mass.

[00:09:59]So, the number of moles of this guy we have how many?

[00:10:02]0.

[00:10:05]uh 02 Now, we need to find the mister. So, iron here has a two.

[00:10:12]So, I'm going to say iron two multiplied by iron the periodic table is supposed to be something like uh 56.

[00:10:21]So, I'm going to say 56 here.

[00:10:23]Then you say two multiplied by 56, you're going to get 112.

[00:10:28]Then oxygen we have three.

[00:10:32]Okay, multiplied by 16.

[00:10:35]So, three multiplied by 16, we're getting 48.

[00:10:40]48 + 112, we're getting 160.

[00:10:45]So, you put your 160 here.

[00:10:47]And then we see what we're going to find. Multiplied by 0.

[00:10:52]point 0.02 So, we're getting a 3.

[00:10:59]2 g 3.2 g of iron three sulfate.

[00:11:06]Sorry, iron three oxide.

[00:11:09]Okay.

[00:11:10]We have calculated it.

[00:11:12]The next question is saying the percentage purity of iron three oxide assuming that the impurities did not react with the acid. So, the percentage purity in this case is going to equal to you're going to get what you calculated which is your mass of the pure substance.

[00:11:37]Okay, so you're going to get your mass of pure substance all over by the mass of impure substance multiplied by 100. So, what was the mass of the pure substance? We calculated it to be 2 3.2.

[00:12:05]Then, what did we have as a mass of the impure substance?

[00:12:08]They were actually saying 4 g of impure iron 3.

[00:12:13]So, you're going to put your four here, then you multiply by 100.

[00:12:19]So, divide by four, multiply by 100, you're going to get 80%.

[00:12:30]All right, we go to the next question.

[00:12:32]Define what a mole is. So, basically, a mole is the amount the amount of substance that contains as many elementary entities as many elementary as many elementary entities as they are in Okay, as they are in exactly carbon 12. Simple.

[00:13:25]Simply, the amount of substance that contains as many elementary entities as they are in exactly carbon 12.

[00:13:33]And that And you get it correct. Now, that was just a definition, actually.

[00:13:37]Okay?

[00:13:38]The question is saying methane steam reforming is the reaction between methane and the steam to produce what water to produce hydrogen gas and carbon monoxide gas. Construct a balanced chemical equation include state symbols.

[00:13:55]Okay. So, basically, they just want you to show what happens when methane which is natural gas, it's a gas, reacts with vapor.

[00:14:07]Of course, what is going to be produced is going to be carbon monoxide plus water.

[00:14:17]Okay?

[00:14:18]Now, let's check what we have. How many hydrogen do we have this side? This side, we have about six hydrogens.

[00:14:24]Well, we can put a three here, there.

[00:14:26]It means we're going to have uh six hydrogens. This side, we have two oxygens. This side, we have one oxygen.

[00:14:33]This side, we have one oxygen this side.

[00:14:35]Okay, so, what do we need? We need to put a two there.

[00:14:41]Okay? We need a two put a two.

[00:14:45]Then, let's see what is going to happen.

[00:14:51]Sorry.

[00:14:52]I think there's a mistake somewhere. Why am I putting water?

[00:15:00]It's supposed to be hydrogen gas. Wow.

[00:15:02]It's supposed to be hydrogen gas, not water. Water is already in the equation.

[00:15:06]So, now, let's check how balanced this is.

[00:15:10]We have two hydrogen, two hydrogen on the other end.

[00:15:14]We have one oxygen this side. We have one oxygen on the other end. Let me check for hydrogens. Actually, we have six hydrogens this side.

[00:15:22]Six hydrogens on our left.

[00:15:26]This side we only have two.

[00:15:28]And that opens our minds because we're just going to put a three here.

[00:15:32]And this is balanced. Okay? That's why I say what I left it. Just know the concept. Just know what you are doing and know what needs to be done. Okay?

[00:15:43]Let's proceed. We see how beautiful this is going to end.

[00:15:47]Now, what are they saying? They're saying construct a balanced chemical equation, which we have done, including state symbols, of course. We get our two marks.

[00:15:55]C is saying, "Given that 80 g of magnesium oxide reacts with dilute hydrochloric acid according to the equation, uh calculate the mass of magnesium chloride that was formed." This is actually an interesting question.

[00:16:09]And it is actually simple and straightforward. I want you to see what is going to happen.

[00:16:16]You have the equation already, which is balanced. I said you will need to have a balanced equation before every anything.

[00:16:23]So, your balanced equation is already there.

[00:16:26]>> [snorts] >> So, we are going to use this balanced equation. We have our magnesium oxide.

[00:16:31]Then we are comparing to what? Magnesium chloride. That's what we want to find.

[00:16:36]They're saying 80 g of magnesium oxide reacts with dilute hydrochloric acid according to the equation. Calculate the mass of magnesium chloride. So, they want us to calculate the mass of magnesium chloride.

[00:16:50]Okay?

[00:16:51]Now, let's check how many moles of magnesium oxide are there in the equation? There's only one mole. So, you can put your one mole here.

[00:17:05]How about mag- uh magnesium chloride?

[00:17:07]Magnesium chloride, there's only one mole here.

[00:17:11]Now, let's try to find the the moles of magnesium oxide because we have this mass.

[00:17:16]So, the mass of magnesium oxide is 80 g.

[00:17:25]Okay?

[00:17:26]But, we need to find the mister of magnesium oxide.

[00:17:32]So, this is how we're going to find it.

[00:17:37]Magnesium, okay, magnesium oxide we're saying we're going to say one multiplied by Okay, what is magnesium on the periodic table?

[00:17:47]What is magnesium on the periodic table?

[00:17:49]Of course, it's 24.

[00:17:53]So, we're going to get a 24 there, and one oxygen there. So, oxygen it's going to be one multiplied by 16.

[00:18:00]We're going to get 16 there. So, the total is going to be 16 plus uh 24, which is going to be like a 40, of course.

[00:18:09]From there, we can say number of moles is equal to the mass over the mister.

[00:18:16]So, number of moles is equal to What is the mass? The mass is 80 g over 40. So, the number of moles we are finding is 2.0 moles.

[00:18:27]So, you're going to put your 2.0 moles here, and then find the moles of magnesium chloride. You discover that the moles of magnesium chloride will be a 2.0, also.

[00:18:39]Because it was a one-to-one uh mole ratio. Then, they're saying what Calculate the mass of magnesium chloride. Same concept, guys.

[00:18:48]The number of moles is equal to mass over the mister. We want to find the mass, so the mass is going to equal to We have already found the number of moles of magnesium chloride. Let's find the mister. So, magnesium chloride is like this.

[00:19:05]So, magnesium is going to be one mole to be by 24, which is equal to 24.

[00:19:14]The chloride is going to be 2 multiplied by 35.5.

[00:19:19]What is 35.35.5 multiplied by 2?

[00:19:25]Okay, which is going to be like 71.

[00:19:27]Then 71 + 24 is going to be 95.

[00:19:32]So, grams per mole.

[00:19:37]So, I'm going to put your 95 here.

[00:19:41]Multiplied by 2, we have 190.

[00:19:44]So, this is going to be our mass, 190 g.

[00:19:48]Now, I want I want you to make an observation.

[00:19:51]Check what I have done.

[00:19:53]Check also what I did here.

[00:19:56]Okay, I want you to also check what I did here.

[00:20:01]Was there anything different?

[00:20:03]Nothing different. I'm just using the same concept to find these questions and they're actually giving me answers.

[00:20:12]Okay? So, it's actually very simple, guys. Small concept is very simple. You would actually get everything.

[00:20:19]Now, let me show you.

[00:20:23]We have been given this particular question. They're saying that 10.5 g of sodium hydroxide was reacted with dilute sulfuric acid according to the equation.

[00:20:33]They're saying we balance the equation.

[00:20:37]Okay, so let's check. We balance the equation. So, let's check if we put a two here.

[00:20:42]We have two sodium there. We have two sodium.

[00:20:46]We have uh let's check oxygen. Oxygen, we have how many this side? We have This is four plus this one, it's five.

[00:20:55]This side we have four plus this one, it's five. Okay, so oxygen is balanced. Let's check hydrogen. Hydrogen, we have two this side. We have two this side.

[00:21:05]Okay, phosphates. Okay, it's balanced.

[00:21:08]So, we just needed to add like a two there.

[00:21:10]Okay?

[00:21:12]We just needed to add like a two and it is balanced. Let me assess again so that I don't get it wrong. Hydrogen we have two we have four actually.

[00:21:24]Hydrogen we have four.

[00:21:26]Okay, so we have four hydrogen this side.

[00:21:30]And then this side how many do we have?

[00:21:32]We have um two.

[00:21:38]So, how about if I add a two here, what happens?

[00:21:41]So, oxygen we have two plus four is going to be six.

[00:21:47]This side we're going to have four plus two oxygen six. Four hydrogen, four hydrogen.

[00:21:55]Sulphate is balanced and then this side.

[00:21:59]Okay, it's now balanced.

[00:22:01]So, this is going to be our balanced equation.

[00:22:03]It's going to be a two sodium hydroxide.

[00:22:17]Okay, produces sodium sulphate.

[00:22:23]This is balanced. Okay?

[00:22:29]So, when balancing equations make sure you start with what is at the beginning.

[00:22:36]So, I just started with sodium. I balanced it. Then I went to oxygen. By the time I was reaching oxygen, it was already balanced.

[00:22:44]So, be clever as you do this. Now, use the balanced chemical equation to calculate the mass of sodium sulphate. I will use the same concept. I will use the same concept. You will see.

[00:22:56]I'm going to get what I've been given, sodium hydroxide. So, I'm going to get my sodium hydroxide.

[00:23:04]And then they want me to go to calculate uh the mass of sodium sulfate. So, sodium sulfate So, I'm comparing it with the sodium sulfate.

[00:23:18]Right?

[00:23:20]Now, let me use the balanced equation. So, according to the mole ratio, I have two sodium there, which is two moles of sodium.

[00:23:29]Then this side I have an average one mole sodium sulfate. Then let me find the number of moles of sodium. So, sodium it is one multiplied by Okay, let me just get my periodic table.

[00:23:49]Okay, so this is my periodic table. Of course, the periodic table is very very important.

[00:23:55]So, sodium multiplied by this is one multiplied by 23, which is going to be 23.

[00:24:00]And then I'm going to have oxygen, which is one multiplied by 16, which is going to give me 16. Then I'm going to have hydrogen, which is going to be one multiplied by one, which is going to give me one.

[00:24:10]Then I add everything, which is 1 + 16 + 23.

[00:24:16]Notice that I'm getting 40 g per mole.

[00:24:20]Now, let me find the number of moles.

[00:24:22]So, number of moles is going to be mass over the mister.

[00:24:26]So, number of moles is equal to What What was the mass that was given? It was given to be 10.5. So, I'm going to put my 10.5 here, then I put my 40.

[00:24:38]So, it will be 10.5 / 40.

[00:24:43]Then I'm getting 0.

[00:24:45]26 25 moles. So, this is what I'm going to put here.

[00:24:53]Watch, guys. Same concept. I'm just doing the same thing over and over again. I'm not changing anything. Then this is going to be 2x is equal to 0.2625 over two over two.

[00:25:10]Then of course my X is going to equal to if I divide this by two, I'm going to get 0.

[00:25:16]13 125 moles. Moles of what? Sodium sulfate.

[00:25:27]Now, I need to find the Mr of sodium sulfate because I originally that number of moles is equal to mass over Mr. And then they're saying we calculate the mass. So the mass is going to equal to this number of moles.

[00:25:46]multiplied by let's calculate the Mr of sodium sulfate.

[00:25:50]So sodium sulfate sodium we have two so two multiplied by 23.

[00:26:00]Two multiplied by 23 it's 46.

[00:26:05]What else?

[00:26:09]We come to sulfur.

[00:26:13]Sulfur, sorry.

[00:26:18]So sulfur we only have one sulfur there sulfur atom. So sulfur let's check what it is.

[00:26:29]32 So this is going to be 32 also.

[00:26:36]Then oxygen 4 multiplied by 16 4 multiplied by 16 64 So 64 plus 32 plus 46 it's going to be 142 grams per mole.

[00:26:59]So, you will put your 142 here.

[00:27:05]So, 142 multiplied by Okay.

[00:27:12]Ah, okay.

[00:27:21]Okay, so what was the number of moles?

[00:27:24]It was 0.13 25, okay?

[00:27:30]So, this needs to be solved so that the lesson does not get too long.

[00:27:37]Okay?

[00:27:40]So, 142 multiplied by 0.13 125 and I get 18.6 3 g.

[00:27:57]This is the mass of sodium sulfate, guys, which was actually formed. And you get it correct.

[00:28:04]Okay, now you are going to solve this particular question for me. I want you to solve this question.

[00:28:10]B5 You solve B5.

[00:28:18]You also solve B4.

[00:28:21]Unless I submit this question to me, okay? You can submit on 0777 6640 46. Please submit the answers because I've already taught uh percentage yield in the previous science session. Make sure you check it out. The science paper two is a long video, 1-hour video, where I solve the MCQs and also other parts of the sections. Okay? So, make sure you submit this.

[00:28:51]I already taught percentage yield.

[00:28:54]Okay? Same concept, guys.

[00:28:57]Okay? So, this particular question.

[00:29:00]Oh, by the way, they're saying what type of reaction is involved? It's just a neutralization reaction.

[00:29:10]If you found this video interesting and helpful for your exams, consider subscribing and hitting the like button and don't forget to share to others.

[00:29:17]Thank you so much.