CHEMISTRY : LIMITING REACTANT AND PERCENTAGE YIELD %

Added: 2026-05-12

[00:00:00]Welcome to this platform. So today's chemistry lesson we are going to answer a question coming from more concept. Now two of the things that we are going to focus on one determining the limiting reactant or reagent. So we are going to focus on how to determine the limiting reagent and also we are going to look at calculating percentage yield. So we are going to look at also how to determine how to calculate the percentage yield.

[00:00:25]So at the end of the lesson should be able to understand those uh concepts.

[00:00:31]Okay, so we are going to use an exam question to achieve our objective. So let us look at the question.

[00:00:38]So the question reads magnesium oxide react with dilute sulfuric acid.

[00:00:44]Magnesium oxide react with dilute sulfuric acid. Now the first thing that you need to understand is a reaction. So this reaction that you are seeing here magnesium oxide is a base. Okay, this is the metal oxide is the base. Then of course acid uh sulfuric acid we know is an acid. Now the first question here is write the balanced chemical equation for this reaction. Write the balanced chemical equation for the reaction. And I guess up to this point you know how to write the balanced chemical equation.

[00:01:17]But if you do not know surely I'll be able to explain you might pick one or two things. So let us look at how we can write the balanced chemical equation. So to write the balanced chemical equation the first thing that you need to understand is what are the reactants. So the reactants here we have magnesium oxide and the chemical formula of magnesium oxide is MgO and this is a solid.

[00:01:43]Okay, then now we are going to react this one with sulfuric acid so that is the chemical I mean that is sulfuric acid which is in aqua state.

[00:01:54]Now what is going to form here what you're going to form is a salt, magnesium sulfate.

[00:02:01]Magnesium sulfate uh written as such, which is aqueous. Then of course down here I can add plus. Uh or maybe I can just try and um Okay, let me just put and water, okay?

[00:02:17]And water, which is a liquid. So, this one's supposed to be on the other side, please, just a space that uh we do not have. So, that is the reaction that is going to uh be. So, magnesium oxide react with sulfuric acid. This is a base, this is an acid. So, what is produced here is a salt and water, okay?

[00:02:36]So, this is what is going to produce.

[00:02:37]Now, every time you're asked the question, write a balance. This is write a balanced chemical uh equation. So, balance, let us check. Magnesium one, magnesium one. Oxygen one plus this, five. Oxygen four plus that, five.

[00:02:53]Hydrogen two, hydrogen two. So, this itself is already balanced. So, please make sure that you write uh water after magnesium sulfate. So, just like that, the first question is answered. And usually the question like this, when it comes, two marks. And just two marks for free, just like that.

[00:03:12]So, let us look at this second part, which is B. So, this one it says 600 g of magnesium oxide was reacted with 9 uh 80 or that is 980 g of dilute sulfuric acid. Now, what are these? This is the mass for magnesium what? Magnesium oxide. This is the mass for sulfuric acid. The first question here under B is determine the limiting uh reagent in the reaction above. Now, the other question that you should answer is what is the limiting reagent? I know we've had this discussion where we are coming from, but in the case you're coming across this for the first time. So, for the limiting reagent is that simply the reactant that is in short in short supply or the one that runs out uh first. And what is important about this limiting reagent that is the one that determines the amount of product that are produced. The moment it runs out, then also the reaction stops. So, that is what you should understand. So, in other words, the limiting reagent is the one that runs out first or that finishes first. Okay. So, how do we determine it now? Now, since we are dealing with a reactant that runs out first, so the first thing surely you must calculate the number of moles of the reactants. So, we are going to find the number of moles of the reactants.

[00:04:34]So, we are going to find the number of moles of reactants, and what are those?

[00:04:38]Magnesium oxide and sulfuric acid. Now, how do we find number of moles? Surely there are different ways to find number of moles, but here we are given the mass of this one, so which means we are going to use the formula for mass. So, number of moles of magnesium oxide is equals to mass divide molar mass. And the molar mass is simply the mass of mass of a substance that is one mole of the substance. Okay, so what is the number of moles of magnesium? So, the given mass is 600 g. Then, what is the formula mass of magnesium oxide? If you look at magnesium, it's 24, then this is 16. Add these should be able to give us what? 40 g per mole. Okay? Now, we can cancel out things here that this that, then 4 into 6, 1 remainder 2 into 20, that is 5. So, in other words, this one should be able to give us what?

[00:05:43]15 moles, if I'm not mistaken. So, this one is 15 what?

[00:05:50]15 uh moles. Okay. Let me just uh confirm with the calculator, but surely it should be able to give us that one. So, that is 600 600 divide uh 40. Yep, that is a 15.

[00:06:05]Then, on the other side, we are going to find the number of moles of sulfuric acid. Okay. So, what is the number of moles of sulfuric acid? So, equally sulfuric acid, we are given the mass, and of course, we can find the formula mass. So, what is the mass there? The mass given is 980 g. Then, what is the formula mass of sulfuric acid? We have this uh 16 I mean see 4 * 16, that is 64.

[00:06:33]That is the oxygen. Then, sulfur 32 plus that should be able to give us 98 g per mole. And if we are to divide here, what are you going to find? 98 into 98 1, that is 1. So, here is simply 10 uh mole. So, this are number of moles of what? Sulfuric acid.

[00:06:55]Now, what is the next step? After you found their number of moles, but there could be other ways of determining the limiting reagent. But of course, this is one of So, after finding the number of moles now, you should be able to look at Look at this. From a balanced equation, there's one mole of this, one mole of that. Now, if I had to compare their moles, you are going to find that you have more of magnesium what? Magnesium oxide than sulfuric acid. So, therefore, this with a lower number of moles is the limiting reagent. So, you can write there a statement. You can write there a statement that sulfuric sulfuric acid is the limiting. So, sulfuric acid is the limiting reagent. So, it's the limiting reagent.

[00:07:47]So, just like that, you've answered the question.

[00:07:50]Let us look at the other one. Calculate the mass of magnesium sulfate that was obtained at the end of the reaction.

[00:07:59]Calculate the mass. Now, the calculated mass is also known as the theoretical mass. But anyway, how do you calculate it? So, you know that you're calculating the mass of the product, which is magnesium sulfate. So, I can write here magnesium uh sulfate. Now, this one we need to form a ratio with a certain uh with one of these uh reactants. So, which of the reactants we pick? Now, here that is where you need to pay attention.

[00:08:27]So, of these two reactants, the one that we pick is the limiting because it determines the products produced. So, what is the limiting? So, it's sulfuric acid. So, we're going to form the ratio of sulfuric acid to magnesium uh sulfate. The quickest way to find the mass here is to use mass to mass. So, mass of this to mass of that. Now, how do you do it?

[00:08:52]So, we are going to use the formula mass of this one. We treat it as mass with a unit of mass given, which is gram. So, what is the formula mass of sulfuric acid? It's 98 g. Then, what about this one? If you look at magnesium sulfate, this is 64. 32 + 24 should be able to give us 120 g. Be able to confirm.

[00:09:18]So, that is uh 64. 64 is coming by multiplying uh four oxygen, which is 16 each. So, 64 + 32, which is sulfur, + 24, which is uh magnesium. Surely, should be able to give us 120. So, what it means that what it means here that 98 g of sulfuric acid react with magnesium oxide to produce 120 g of magnesium sulfate. So, what about what is the mass of sulfuric acid what is the mass of sulfuric acid? What about 980 of sulfuric acid? What about 980 of sulfuric acid what will be the mass there? Okay, so this one should be able to cross multiply and they should be able to find 98.

[00:10:09]That is 98 g x is equals to 980 g multiplied by 120 g. So, divide 98 g divide 98 g. So, here you should be able to find let me just um Let me just bring it there on top.

[00:10:34]Let me just bring it there on top.

[00:10:37]So, what we what we are able to find So, after dividing there you should be able to find Look at this should be able to find x is equals to we want to find x which is the more I mean the mass of magnesium sulfate. 98 and 98 cancel this is just 10 multiplied by that which is the 1,000 200 g. So, that is the mass of magnesium sulfate produced. Okay, they are saying calculate the mass of that was obtained.

[00:11:16]So, mass of sulfate that was obtained.

[00:11:20]The question extend 1080 or 1,080 g of magnesium sulfate obtained was obtained. Calculate the percentage yield.

[00:11:32]1080 of 1080 g of magnesium sulfate was obtained. Calculate the percentage yield. So, the other question here that we are answering, which is the third question, is percentage yield. How do you find percentage yield? So, percentage yield is equals to actual actual yield.

[00:11:55]So, actual yield divide that is the theo- retical theoretical yield.

[00:12:08]Actual yield divide theoretical yield multiplied by 100.

[00:12:13]That is how you find the percentage yield. Now, let us replace.

[00:12:17]Let us replace.

[00:12:19]So, if I were to replace here, what is the actual yield? Look at this. The amount obtained is percent I mean is actual yield. It's always be given. You cannot calculate the actual yield. So, gram divide what is the calculated mass or what was obtained.

[00:12:35]So, the calculated mass is 1,000 200 g. So, multiply this by 100 after dividing.

[00:12:43]So, what is it going to give us? That is 1080.

[00:12:48]1080 divide 1,200.

[00:12:52]So, if I were to divide this one multiplied by 100 is giving us 90 percent.

[00:12:59]So, 90% is the percentage what?

[00:13:03]Percentage yield.

[00:13:05]What are the questions that you expect here or the questions that you should ask or things to use to verify that what you've calculated is correct?

[00:13:13]So, percentage yield is always less than 100% because a lot of things happen when transferring the reagent or the product.

[00:13:22]You might have lost some of it. Okay?

[00:13:24]So, you cannot have 100% uh percentage yield due to some reasons that I've given. And of course, you should be able to look at some more reasons.

[00:13:33]Aside from that, you should be able to know that the amount obtained or the actual yield is always less than the theoretical. The theoretical is what you think you're going to find. It's the calculated one.

[00:13:43]So, always is greater than the actual yield. Okay?

[00:13:47]So, this is what we can answer when it comes to uh this uh question. So, two of the things that you should put in mind, how to determine the limiting reagent. I guess you've learned how to do that.

[00:14:00]How to calculate the percentage what?

[00:14:02]Percentage yield. So, this is how you do it. Now, go to the questions and look at the question. In case you find out or you find difficult questions, be able to share so that we can answer them together as well.

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