moles heading

Higher tier only

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 of carbon dioxide, CO2 is found by adding up the Ar of carbon and oxygen e.g.

Ar of carbon =12 Ar of oxygen=16
so Mr of CO2 = ( Ar of C) + (2x Ar of oxygen)
Mr= 12 + 32 =44.
Also 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 of carbon dioxide is 44g.
If 44g of C02 = 1 mole of C02, then how many moles is say 11g of C02 ?
well: 1 mole of C02 = 44g so
0.5 moles of C02 = 22g so
0.25 moles of C02 = 11g
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 substance.

glucose molecule

Example 2

The molecule shown opposite is glucose sugar, its molecular formula is C6H12O6.
The relative formula mass of glucose, C6H12O6 is found by adding up the 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:

This leads up to 3 formula for calculating the:

Example
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.

Check your understanding - complete the following practice questions

Next