Hi,

You will never need a exact mass value like you Mass of hydorgen is considerd as 1 but it is 1.004g almost

Trick = Most of the time you can just double the value atomic number you will get the mass

example = oxygen has 8 atomic number so mass will be 2 x 8 = 16

carbon has 6 atomic number just double it 2 x 6 = 12g

Like that,

Thanks