Relative formula mass
Finding the relative formula mass is simple. All you have to do is add up the masses of each atom in the compound or molecule.
E.g. Carbon dioxide.
        CO2
       Carbon has a molecular mass of 12 and Oxygen has a molecular mass of 16.
There are 2 oxygens so we do (16*2)+12=44

Another example is water.
        H2O      The sum for this would be (1*2)+16=18

Empirical Formula
Empirical formula tells us the ratio of each element within a compound. To work it out we must first find the number of moles, we do this by dividing the mass (or percentage) of each element and dividing but it molecular mass.
E.g. Carbon Dioxide.
       In a compound there contains 12g of carbon and 32g of oxygen, work out the formula.
       First of all we divide the mass by the molecular mass:
       12g/12=1mol 32g/16=2mol we divide both the numbers by the smallest, which is 1, to find the        ratio. In this case the ratio is 1:2 so the compound is CO2

Percentage yield
Percentage yield tells us what percentage of product was formed compared to what we should have received  from the reaction. We don't always get 100% due to impure compounds and human errors.

%yield=(Actual yield/ theoretical yield)*100
E.g. Johnny thermally decomposes 60g of CaCO3 which give s him 15g of  CaO, find the percentage yield.
To do this we first need to work out how many grams of CaO should have been produced.
Write down the equation to help:
CaCO3->CaO+CO2
We need to find out the number of moles in CaCO3 60/100=0.6moles
Because it's a 1:1 ratio CaO will also contain 0.6moles. We then find the mass by multiplying moles by molecular mass. 0.6* 56=33.6g (our theoretical amount)
We then do (15/33.6)*100=44.6%
That's the final answer.