moles heading

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If you are not sure what the mole or the relative atomic mass (Ar) are or you are not sure how to calculate the relative formula mass (Mr) of a substance then click here before you read this page.

Calculating the number of moles

molecule of carbon dioxideTo calculate the relative formula mass, Mr of a compound you simply add up the relative atomic masses, Ar of the elements that make it up. The relative formula mass (Mr) of carbon dioxide, CO2 is found by adding up the relative atomic masses (Ar) of carbon and oxygen e.g.

Ar of carbon =12     Ar of oxygen=16
so the relative formula mass Mr) of CO2 = (Ar of C) + (2x Ar of oxygen)
Mr= 12 + 32 =44.
Also we can say that the mass of 1 mole is just the Mr expressed in grams. So 1 mole of CO2=44g.
This also means the mass of 6x1023 molecules (that is Avogadros number of molecules) of carbon dioxide is 44g.
If 44g of C02 = 1 mole of C02, then how many moles is say 11g of C02 ?
well:

You may have worked this out in your head quickly in one step by dividing 11 by 44. If you did then you used the formula below:

Mole calculation formula

These simple formula allow you to calculate the number of moles of any element or compound present.

glucose molecule

Example 2

The molecule shown opposite is the sugar glucose, its molecular formula is C6H12O6.
The relative formula mass (Mr) of glucose (C6H12O6) is found by adding up the relative atomic masses (Ar) of carbon, hydrogen and oxygen just as we did above for carbon dioxide e.g.

Ar of carbon=12,   Ar of hydrogen=1,     Ar of oxygen=16
so Mr of glucose, C6H12O6 =(6x Ar of C) + (12x Ar of hydrogen) + (6x Ar of oxygen)
Mr= 72 + 12 + 96 =180.
Also the mass of 1 mole is just the Mr expressed in grams. So 1 mole of glucose, C6H12O6 =180g.

So if we had, say 37g of glucose, how many moles of glucose would this be?
Well using the formula above we have:

In mole calculations:

You should be able to use the formula below to calculate either:

All that is needed is a bit of simple arithmetic and to rearrange any formula given. The highlighted box below shows all three formulae which you could be asked to use.

Example 3

What is the mass of 0.65 moles of glucose? To calculate the mass we use the formula :
mass = mumber of moles x Mr
m= n x Mr
m = 0.65 mol x 180 = 117g
so 0.65 moles of glucose has a mass of 117g.

Key points