Higher tier only
To 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
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:
In mole calculations:
This leads up to 3 formula for calculating the:
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.