Volume Strength of Hydrogen Peroxide

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. You pick up a bottle of hydrogen peroxide, and the label says: twenty volume. Twenty volume? Twenty what? The bottle holds one litre of liquid, right? Here is the surprise. That one litre of liquid can give twenty litres of oxygen gas! But how? And how much hydrogen peroxide is really inside that bottle? By the end of this video, you will read any such label in ten seconds. And look, JEE loves this label. It can ask it as molarity, as normality, or as grams per litre. So, let's begin.

0:39What does “x-volume” mean?

So, first, what does this label mean? Hydrogen peroxide, H two O two, comes as a solution, and its strength is written as x volume, like ten volume or twenty volume. Any guess what that number is telling you? Yes, it is about oxygen! x volume means: one litre of the solution gives x litres of oxygen gas at STP, when it decomposes. STP on screen means standard temperature and pressure, okay? So twenty volume means one litre gives twenty litres of oxygen. That is the first half of our puzzle, solved in one line. Clear? Now, where does this oxygen come from? Hydrogen peroxide decomposes: two H two O two give two H two O, plus one O two. Look carefully at the numbers in front. Two moles of peroxide, but only one mole of oxygen, okay? Remember this two is to one. It is the key to the whole video. And one more small thing. The ratio stays the same for any amount. One millilitre of x volume solution gives x millilitres of oxygen. So for a bigger amount, you just multiply. Really simple.

1:49From litres of oxygen to molarity

Alright, now the real question. How much hydrogen peroxide is inside one litre? Start from the oxygen. At STP, we take one mole of gas as twenty two point four litres, okay? So x litres of oxygen is x over twenty two point four moles. And the solution is one litre. So the molarity of hydrogen peroxide is x over twenty two point four, right? No, no, careful! This is the JEE trap. Those are moles of oxygen, not moles of peroxide. Go back to the equation. For every one mole of oxygen, how many moles of peroxide broke down? Two, exactly! Two moles of H two O two give one mole of O two. So the moles of peroxide are twice the moles of oxygen. Two times x over twenty two point four. And twenty two point four divided by two is eleven point two. So, x over eleven point two moles, in one litre of solution. And moles in one litre is exactly the molarity. Molarity equals x over eleven point two. Write this down. This one line is the heart of today's lesson. Got it?

3:04Normality and strength from the same number

Now, from this one formula, everything else follows. Molarity, x over eleven point two, is our starting point. Keep it in front of you, okay? We will build two more answers on top of it. The normality of x volume peroxide is x over five point six. Now remember, normality equals molarity times the n-factor. So compare x over five point six with x over eleven point two. What is the n-factor here? Two, absolutely! Five point six is eleven point two divided by two, so the normality is exactly twice the molarity. So for hydrogen peroxide here, the n-factor is two. With me? Next, strength in grams per litre. Moles times molar mass gives grams, right? And the molar mass of H two O two is thirty four grams per mole. So strength equals thirty four x over eleven point two grams per litre. Simply thirty four times the molarity. Now look, one more JEE trap. Some questions take the molar volume as twenty two point seven litres per mole, not twenty two point four. Then what changes? Half of twenty two point seven is eleven point three five. So replace eleven point two by eleven point three five, okay? Always read which molar volume the question gives, before you divide.

4:30Topper's trick: the halving ladder

Now let me share a trick that toppers use. You don't need to mug up three formulas. Just remember one ladder. Twenty two point four, halve it, eleven point two, halve again, five point six. Molar volume, then molarity, then normality. And going backwards? If the question gives you the molarity and asks for the volume strength, what do you do? Just multiply, that's right! x equals eleven point two times the molarity, or five point six times the normality. Capital M is molarity and capital N is normality, okay? Quick and easy. And for grams per litre, find the molarity first, then multiply by thirty four. That's it, three numbers, and every volume strength question is done. Got it?

5:21Example 1: volume strength of 1.5 N H2O2

Right, let's try one. An easy one first. What is the volume strength of one point five normal hydrogen peroxide? Careful: normal, not molar. Pause the video, and try it first. Done? Okay, let's check. Which rung of the ladder do we need? The question gives normality, so we use five point six. Normality equals x over five point six, so x equals five point six times the normality. Five point six times one point five. That is five point six, plus half of five point six, which is two point eight. Eight point four. So the answer is eight point four volume. One litre of this solution gives eight point four litres of oxygen at STP, okay? And if you multiplied by eleven point two? You used the molarity rung for a normality, and your answer came out double. That is the trap, right? Always check first: is it molar or normal?

6:24Example 2: molarity and strength of 20 volume H2O2

Now a slightly harder one, JEE pattern. And this is our bottle from the start! Find the molarity, and the strength in grams per litre, of twenty volume hydrogen peroxide. Pause and try. Hint: start from the oxygen. Done? Okay, let's check. Twenty volume means one litre of solution gives twenty litres of oxygen at STP, okay? So that is twenty over twenty two point four moles of oxygen. So the molarity is twenty over twenty two point four, right? No, no, wait! That is the same trap we saw. These are moles of oxygen. What about the peroxide? Two moles of peroxide for every mole of oxygen, exactly! So the peroxide is two times twenty over twenty two point four, that is forty over twenty two point four. One point seven eight six moles, in every litre. So the molarity is one point seven eight six, about one point seven nine molar. Quick check with our ladder: x over eleven point two. Twenty over eleven point two also gives one point seven eight six. Same answer, and in just one step! Now the strength: molarity times thirty four. One point seven eight six times thirty four is sixty point seven grams per litre. So, our twenty volume bottle has sixty point seven grams of hydrogen peroxide in every litre, and that litre can give twenty litres of oxygen. Puzzle solved!

7:57Quick revision: M = x/11.2, N = x/5.6, strength = 34M

So, quick revision. x volume means one litre gives x litres of oxygen at STP, okay? Molarity is x over eleven point two, normality is x over five point six, and strength is thirty four times the molarity, in grams per litre. And if the molar volume is twenty two point seven, use eleven point three five in place of eleven point two. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!