# Mole concept Molar mass and mass no

1. May 30, 2015

### Brajesh kedia

The most so called "difficult" topic in chemistry mole.I am Confused in it a lot..please clear the doubt.
Firstly,The mass no of each elements means the mass Of how much atoms in the elements and relative to what.as we know we assume a prototype as scientist did named it as kilogram and then on relative to that we find mass of everything..

2. May 30, 2015

### Staff: Mentor

Mole is just an overgrown dozen - 6.02×1023 of atoms (or molecules, or ions, or whatever).

We did some tricks to make calculation of mass of the mole trivial, don't let them muddy the water.

3. May 30, 2015

### Brajesh kedia

I know this borek ji.First of all i would like to clear wid the doubt i asked you above so that i cant match my concepts according and if still confused will confirm those doubts

4. May 30, 2015

### epenguin

They didn't know what that number was when they invented that concept of mole. You can understand moles without knowing this number. It was just the number of atoms in a gram of hydrogen, even when we didn't knlwnwhat that number is. It still is! - almost. (Best to ignore the refinements if you're having such difficulties.) Different atoms combine with each other to form compounds with (in the beginning) small whole numbers of atoms of different elements, e.g. water has one atom of oxygen and two of hydrogen, methane has one of carbon and four of hydrogen. But the atoms of hydrogen, oxygen and carbon all have different masses, that of oxygen is about.16 times that of hydrogen, of carbon about 12 times that of hydrogen. So water combines two grams of hydrogen to one of oxygen so by grams they combine about 2:16 = 1:8. In grams, methane is about 4:12 = 1:3 in grams. The same numbers carry over to other compounds so the combination to make carbons dioxide (one atom carbon and two oxygen) is 12:2×16 = 1:1.66 . The combining numbers are easy to immagine - but they are not easily visibile in the laboratory - what you could measure in laboratories was combining weights. Still true for what people mostly do in chemistry. Hence all this science of stoichiometry, and all the calculations about it in excercises.

Which many students seem to have enormous difficulty with. Yet they involve nothing but simple proportions. So I wonder why the difficulty? I guess it is because one science is called chemistry and the other is arithmetic. And when they do one they can't carry the other over and mix them.

Another related problem is perhaps the rather algebraic-like notation for chemical compounds. Maybe if they wrote coloured blobs for atoms, say a white one for hydrogen a black on for oxygen and remembered the White on weighs 1 and the black one 16 they would get it.

However I think what I said in the first para is in your textbook so I hope re-reading it you can undesrstand it there, and bear in mind you can't expect us to write the textbooks again.

5. May 30, 2015

### Brajesh kedia

What do you mean by gram of hydrogen...do you mean it has molecular mass 1 then 1g mass and what is the prototype for comparison of the atomic weight.As i knw C-12 atom which itself has mass how is that possible?? Confused

Last edited by a moderator: May 31, 2015
6. May 30, 2015

### Entanglement

7. May 31, 2015

### epenguin

What I mean by a gram of hydrogen is just that - 1 gram lf hydrogen. Hydrogen is a substance you can weigh. A gram is a unit of weight, arbitrary but standardised. A mole is the amount of hydrogen atoms in gram of hydrogen. (It used to be called a gram-atom.) It made sense to use hydrogen as the basis becuase it is the lightest atom.

To take an even simpler example than before: hydrogen can combine with bromine to form a compound hydrogen bromide. This contains one atom of hydrogen and one of bromine. The chemical symbol of it is H-Br or HBr. The bromine atom is about 80 times heavier than the hydrogen atom. So one mole of bromine atoms (I.e. the same number of atoms as in 1g or 1 mole of hydrogen) is about 80g; one mole of HBr about 81g.

Your point about 12C is the refinement I said best ignored till the concept is clear. The reference substance has been changed by standards authorities for practical purposes but 1g hydrogen is still 1 mole and vice versa to within less than 1%, which is as good as you'll need for most calculations.

Last edited: Jun 1, 2015
8. May 31, 2015

### Almeisan

Say you use two apples, 100 grams of raisins and 1 spoon of sugar in your apple pie recipe.

You measure by counting, by weighing mass and my measuring volume. Three different ways to determine an amount.
Moles are the counting. You just add 23 zero's to make 1 mole something you can manage in the lab.

How is dragging up history making it straightforward?