Calculations

To calculate the Relative Formula Mass (Mr) you add up the Ar values

 e.g. CO2  C=12   0=16        12 + 16x2 = 44

To calculate the Percentage Mass using the Mr:

Ar x Number of atoms divided by total Mr x 100

E.g. What is the percent of Carbon in CO2?    Ar values C=14 O=16

14 divided by 44 x100 = 31.81%

To work out the formula of a compound:

E.g. I react 44.8g of Iron with 19.2g of Oxygen, what is the formula?

Fe         O

44.8      19.2          (Mass given in question)

56         16  …