Molecular Mass and Formula Mass

Transcript

0:00Introduction

Hello everyone! Okay, one quick question before we start. What is the molecular mass of sodium chloride, NACL? You have already started adding numbers, right? Hold on, wait. Trick question! Solid sodium chloride has no separate molecules at all. So then, what exactly are we calculating? And a second puzzle: copper sulphate crystals, CUSO four dot five H two O, carry water inside them. Does that water count in the mass? By the end of this video, you will answer both. And look, almost every mole calculation in JEE chemistry starts with this one step: finding the mass of one molecule, or one formula. So, let's begin.

0:48Molecular mass: add up every atom

So, the definition first. The molecular mass is the sum of the atomic masses of all the atoms in one molecule, okay? Nothing more. Just add them up. And the unit is the same u, the atomic mass unit, from our last video. But careful: we add every atom, not just every element. So for each element, take the number of its atoms, the small subscript in the formula, multiply by its atomic mass, and then add everything up. Clear? Now, you try one. Water, H two O, okay? Hydrogen is one, and oxygen is sixteen. What is the molecular mass of water? Yes, eighteen! Two hydrogen atoms at one each give two, plus one oxygen at sixteen. Eighteen u. And if you said seventeen? You counted hydrogen only once. See how much that little subscript two matters?

1:40No molecules? Then it is the formula mass

Now, back to our first puzzle. Sodium chloride is an ionic solid. And in an ionic solid, there are no discrete molecules: no separate little units that you can pick out one by one, okay? So the word molecule does not really fit here. So what do we do now? Throw the whole idea away? No, no! We do exactly the same addition. We add the atomic masses of the atoms in the formula, NACL, the same sum as before. Only the name changes, and that is what the arrow on screen says: for ionic solids, we call this sum the formula mass. So that is the answer to our hook: for sodium chloride, we calculate its formula mass. Got it? So, the rule is simple. If the substance is made of molecules, the sum is called the molecular mass. If it is an ionic solid, the same sum is called the formula mass. The calculation is identical. That's it, only the name is different.

2:38JEE trap: water of crystallisation counts

Now the second puzzle, and this is a favourite JEE trap. Look at the formula on screen: CUSO four, dot, five H two O, okay? The water written after the dot is called water of crystallisation. So, when we find the formula mass, does this water count? Many students simply ignore everything after the dot. Some add only one water, just eighteen. No, no, careful! Both are wrong. Water of crystallisation is included, always. It is part of the formula. That small dot, which you see on screen, does not mean multiply, okay? It simply joins the water to the copper sulphate. And the five in front means five whole water molecules. So we add five times eighteen, that is ninety u. Not eighteen, and not zero. Ninety. Remember this one; examiners love it.

3:34Example 1: masses of glucose and CuSO4.5H2O

Alright, time for the first example. Find the molecular mass of glucose, C six H twelve O six, and the formula mass of CUSO four dot five H two O, our copper sulphate. Carbon is twelve, hydrogen one, oxygen sixteen, sulphur thirty two, and copper sixty three point five. Pause the video, and try it first. Done? Okay, let's check. Glucose first. Six carbons at twelve each: seventy two. Twelve hydrogens at one each: twelve. Six oxygens at sixteen each: ninety six. Seventy two plus twelve plus ninety six: one hundred and eighty u. Nice and clean. Now copper sulphate. Copper is sixty three point five, and sulphur is thirty two. And oxygen? How many oxygen atoms are there in CUSO four? Yes, four! So, sixty three point five for copper, plus thirty two for sulphur, plus four times sixteen for the oxygens. And then, remember the trap: plus five times eighteen, for the five waters. With me? So, sixty three point five, plus thirty two, plus sixty four, plus ninety. Two hundred and forty nine point five u. And if you got one hundred and fifty nine point five? You forgot the water, didn't you? That is exactly the trap we just saw.

4:56Example 2: S8 molecular mass and molecules in 64 g

Now a slightly harder one, JEE level. Rhombic sulphur exists as S eight rings: eight sulphur atoms in one ring, which is one molecule. Find its molecular mass, and the number of molecules in sixty four grams of it. Sulphur is thirty two. Pause the video and try it. Alright, molecular mass first. Sulphur is thirty two, so the answer is thirty two, right? No, no! Thirty two is the mass of one sulphur atom. Our molecule has eight of them. So, M, the molecular mass, is eight times thirty two: two hundred and fifty six u. See? Even an element has a molecular mass, when its atoms join to form molecules. Now the number of molecules. Take the sixty four grams and divide by two hundred and fifty six: sixty four over two hundred and fifty six, the fraction on screen. That gives zero point two five mole of S eight, okay? We will study the mole properly in the next video. For now, just follow along. Then multiply by the Avogadro constant, six point zero two two times ten to the power twenty three. Zero point two five times that: one point five one times ten to the power twenty three molecules. And one last check, a classic JEE trick. How many moles of sulphur atoms is that? Still zero point two five? No, careful! Each molecule has eight atoms. So zero point two five times eight: two moles of sulphur atoms. Molecules and atoms are not the same count. Always read which one the question asks for, got it?

6:42Quick revision: molecular mass, formula mass, hydrates

So, quick revision. Molecular mass: add the atomic masses of every atom in the molecule. Ionic solids like sodium chloride have no discrete molecules, so the same sum is called the formula mass. And water of crystallisation always counts: copper sulphate with five waters has five H two O in its mass. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!