The+mole+concept

//So this is the first lesson and here we go.//

**The mole concept and Avogadro’s constant**

We know that every thing is made up of atoms. These atoms are so small that it is very hard to count them. Trying to count the number of water molecules in a glass of water would be as hard as trying to count the number of grains of sand in a bucket filled with sand.

So what do we do?? Obviously chemists need to find a way of measuring the number of atoms or molecules present in a substance so that they can make calculations especially if they want to know how much of a product is made in a reaction. So they came up with a system of measurement called the mole.

One mole is that amount of a substance that contains 6.02 X 10 23 number of particles. Particles could be atoms/ions/molecules. In simple terms it means that every 6.02 X 10 23 particles make up one mole. So if you have 6.02 X 10 23 number of water molecules, you can say that you have a mole of water molecules. Using moles is like using the kilogram. It is ju st a way of measuring the number of particles. So We can now conclude that:

1 mole of particles (water molecules) = 6.02 X 10 23 of the particles (water molecules)

2 moles of particles = 2 X (6.02 X 10 23 ) = 1.204 X 10 24 of the particles

3 moles of particles = 3 X (6.02 X 10 23 ) = 1.806 X 10 24 of the particles

//And// //so on…// // …. //

Note:
 * //The number 6.02 X 10 23 is called Avogadro’s constant since it was discovered by the scientist Avogadro.//
 * // 'Mol' is a short form for writing 'mole'!!! //

// So now let’s do a bit of calculations: //

If a sample contains 0.5 moles of ammonia (NH 3 ): We know that 1 mol = 6.02 X 10 23 molecules so, 0.5 mol = 0.5 X 6.02 X 10 23 NH 3 molecules. It is just another way of saying that half a mol (0.5 mol) contains half the number of particles that one mole contains.
 * **//How many ammonia molecules do we have?//**

We know that we have 0.5 X 6.02 X 10 23 NH 3 molecules and each NH 3 molecule contains 3 H atoms so the number of H atoms will be = 0.5 X 6.02 X 10^23 X 3
 * **//How many hydrogen atoms do we have in the sample?//**

Exactly. We have 0.5 X 6.02 X 10 23 X 1 N molecules.
 * **//I am sure you can now tell me how many nitrogen molecules we have?//**

// **Good Job!!** //

Now try some of the following on your own:

A sample contains 0.1 mol of aluminium (III) sulphate Al 2 (SO 4 ) 3.
 * 1) How many Al 2 (SO 4 ) 3 molecules do we have?
 * 2) How many Al 3+ ions do we have?
 * 3) How many SO 4 2- ions do we have?

Attempt the questions and then Check the answers to the questions to see if you have got it right 1) 0.1 X 6.02 X 10 23 molecules 2 &3) We know that Al 2 (SO 4 ) 3 is an ionic compound and that is why we talk about the number of Al 3+ ions and SO 4 2- ions. So we know form the formula that each Al 2 (SO 4 ) 3 molecule contains 2 moles of Al 3+ ions and 3 moles of SO 4 2- ions. - So we have the number of Al 3+ ions = 0.1 X 6.02 X 10 23 X 2 - And number of SO 4 2- ions = 0.1 X 6.02 X 10 23 X 3

Well if you feel that you are comfortable with this unit, I suggest you move on to the next. By end of this unit you should:
 * **Know that one mole of a substance contains 6.02 X 10^23 molecules **
 * **Be able to convert between number of moles and the number of molecules/ions in a substance **

J If you have checked both of the above, then it is time to move to the next unit.

media type="facebooklike" key="http%3A%2F%2Fibchem4u.wikispaces.com%2FUnit1.1-mole%20concept" width="270" height="48"

The following are the sources of the photos: Sand bucket-- ([] ) 08Feb2012 Glass of Water--( [] ) 08Feb2012