Gay Lussac's law of combining volume #Gasvolumes#volumeratio#Reactingvolume#

Added: 2026-05-26

[00:00:00]Hi, welcome to Sarasi Science Hope.

[00:00:04]Today we'll be looking at Galuzzac's law of combining volume. Galuzzac's law of combining volume says that when gases react, they do so in volumes which are in simple ratio to one another and to the volume of their product if gous provided that temperature and pressure remain constant. Let me explain. When gases react, note the reactants must be gases. And when these gases react, they do so in volume. They react by volume. They combine by volume, not by mass. And the volumes of these reactants are in simple ratio. What do I mean by simple ratio?

[00:00:41]Small whole numbers with no fractions or decimal. The volumes of these reactants are also in simple ratio to the volume of their products. if gaseous provided that temperature and pressure remain constant. Let's look at this equation for a better understanding. This reaction between hydrogen and oxygen.

[00:01:02]Look at the ratio. These are small whole numbers. Two ratio 1 ratio 2. Small whole numbers. They are not fractions.

[00:01:10]They are not decimals. So when gases react, they react by volume which are in simple ratio. Which are in small whole numbers. Okay to one another and also to the volume of their product if gaseous provided that the temperature and pressure remain constant. Let's look at this jam question.

[00:01:30]60 cm cube of hydrogen are sparked to 20 cm cube of oxygen at 100° and one atmosphere. The total volume of residual gases is not while solving we have no business with temperature and pressure. So don't let it confuse you.

[00:01:49]What's the first step? Just listen to the steps and you'll be able to solve these questions carefully. Questions under Gilluzak law of combining volume.

[00:01:58]What's the first step? Write out the equation for the reaction.

[00:02:07]This is the reaction between hydrogen and oxygen. You know that hydrogen exists as a diatomic element. That's hydrogen alongside oxygen to give us water. Right?

[00:02:19]And make sure your equation is balanced.

[00:02:25]I have two atoms of hydrogen here. I have two atoms of hydrogen here. Oxygen 2. Oxygen here. One. The equation must be balanced. Okay. To balance this equation.

[00:02:40]I have four here, four, oxygen 2, then oxygen here. So equation balance.

[00:02:45]Remember I said the first step is to write out the equation for the reaction which I've I've just done. Now what's the second step? Write out the mole ratio.

[00:02:59]What's the mole ratio here? I have two.

[00:03:01]What do I have here? One. And what do I have here? Two. Now note mole ratio in gas equation can be represented as volume ratio. Okay? And remember Gilluzak says that when gases react they do so in volume. So this more ratios can also be represented as volume ratio that I can write two here.

[00:03:30]Very simple. What's the first step?

[00:03:32]Write out the equation for the reaction.

[00:03:34]What's the second step? Write out the mole ratio. And what is the third step?

[00:03:39]Represent the mole ratio with what?

[00:03:40]volume ratio. Then the next step is to write out the given volume. The given volume is from the question.

[00:03:49]Now let's travel down to the question.

[00:03:51]What's the given volume of hydrogen 60 cm cq?

[00:03:56]Right? Given volume of oxygen 20 cm cq for the products we do not know.

[00:04:04]Now note that you have 60 cm cube of hydrogen in the question does not mean that all of it will react and that you have 20 cm cube of oxygen does not also mean that all of it will react. It is your duty to get the reacting volume.

[00:04:20]And how do we get the reacting volume?

[00:04:22]Just pay attention.

[00:04:26]Reacting volume how do you get the reacting volume?

[00:04:31]Let's call this case one.

[00:04:35]Write that. Write out the volume of the reactants from the equation. The volume of the reactants from the equation. From the equation, this is two.

[00:04:45]Hydrogen is one, right? Good. The second step, write out the given volume of one of the reactants and then represent the given volume of the other reactants with a variable. I'm choosing hydrogen first, which is 60. Then I'll represent the volume of oxygen with a variable which is x. So when I cross multiply 60 will be equal to 2x when I make s subject of formula 60 / 2 to give me 30.

[00:05:15]I will repeat this step. But in case two I'll write out the given volume of oxygen then represent hydrogen with a variable.

[00:05:25]Case two, volume from the equation. Now I'm representing hydrogen with a variable.

[00:05:35]Then I'll write out the given volume of oxygen. So when I make s subject of formula 40 cm cube. Now note For oxygen I got 30 cm cq. Now the question is it is this is it possible to have 20 cm cq of oxygen and then the reactive volume will be 30 it is practically impossible. Okay but that of hydrogen we have 60 cm cq of hydrogen and then from what we solved 40 cm cq of hydrogen which is lesser than the given volume.

[00:06:23]So we can't use this. Why? Because this is higher than the given volume in the question. Okay? When you solve for the reacting volume, if you get a lesser value, you make use of it.

[00:06:41]Like I said earlier on that you have 60 cm cube of hydrogen does not mean all of it will react and that you have 20 cm cube does not mean all of it will react.

[00:06:50]for you to know if all of it will react, you need to solve for the reacting volume. Okay, which we've done. And I said if you get a lesser value compared to the one you have in the question, you make use of it. So for 60 for hydrogen, what did we get? 40 cm cube.

[00:07:09]When we solve for oxygen, we got a higher value. Okay, the given volume. So we still make use of 20.

[00:07:19]It simply means that the 20 cm cube of oxygen will react. Now, how do we get the volume of the product? Very simple.

[00:07:28]Which of the reacting volume of the reactants did we solve for? We solved for hydrogen, right? Remember, we're not making use of the one we solved for oxygen and I told you the reason we solved for hydrogen. So, we compare the volume ratio from the equation.

[00:07:44]Two of hydrogen will give us two of water right what will 40 cm cube of hydrogen give us x when I cross multiply 80 will be equal to 2x that's 40 40 * 2 will give me 80 will be equal to 2x when I make s of formula have 8 / 2 to give me 40 cm cq so it simply means that the volume of the product is 40 cm cube.

[00:08:20]Okay. Now, if 40 cm cq of hydrogen will react, what's the leftover gas? 60 - 40 to give me 20 cm cube. Here I will have zero because all of it will react. And for the products, we have what? 40 cm cube. Now let's go back to the question. What's the question? Total volume of residual gases. Gases not one gas. And how would you get the total volume of residual gases?

[00:08:55]Volume of the products plus volume of the unreacted gas.

[00:09:07]Total volume of residual gases.

[00:09:16]Volume of unreacted gas plus volume of the products.

[00:09:33]What's volume of unreacted gas? 20 cm cube. Right.

[00:09:39]That's volume of leftover gas plus 40 cm cube volume of the product to give us 60 cm cube. Do we have 60 cm cube in the option? Yes, that's option A.

[00:09:54]Bye and see you in my next class.