Unit 4 - RAM, Moles and equation revision notes

CHEMISTRY: HOW MUCH (CALCULATIONS)

Relative Formula Mass (Mr)

To find the relative formula mass of a compound you add up the relative atomic
masses of all the atoms in the compound.

e.g. MgSO4 contains:

1 x Mg: 1 x 24 = 24

1 x S: 1 x 32 = 32

4 x O: 4 x 16 = 64

So the relative formula mass = 24 + 32 + 64 = 120




Moles

The relative formula mass of a substance in grams is known as ONE MOLE of
that substance.

e.g. MgSO4 Relative Formula Mass = 120

Mass of One Mole = 120grams

A mole of a substance always contains the same number of particles, just like a
dozen of anything is always 12.




Calculating the Percentage Mass of an Element in a Compound

e.g. Find the mass of hydrogen in CH4

1. Write down the formula: CH4



2. Work out the relative formula mass:

, C= 12

H= 1

H=1

H=1

H=1 relative formula mass = 12 + 1 + 1 +1 + 1 = 16




3. Work out the mass of the element you are looking for in one formula
unit (e.g. the amount of H in CH4)




CH4 : there are four hydrogens in the formula,

so the mass is 1 + 1 + 1 + 1 = 4 (1 is the atomic mass of hydrogen)



4. Calculate the percentage of the element in formula



Mass of the element you are looking for x 100

The relative formula mass of the entire compound

4 x 100 = 25 %
16



Working Out the Formula of a Compound from Reacting Masses
(or the EMPIRICAL formula)

e.g. A sample of a compound has 16g of sulfer and 24g of oxygen. Work out the
formula of this compound.

It is easiest to do this in a table: