4.4.1 Relative formula mass

Question 1

What is RFM and how is it calculated?

Answer 1

Relative formula mass is the mass of a formula unit of a substance. It is calculated by adding the RAMs of all the atoms or ions.
e.g. RFM of H2O:
RFM = (2x1) + 16 = 18

Question 2

What is percentage by mass and how is it calculated?

Answer 2

Percentage by mass shows the mass of a substance that comes from a particular element. It is calculated from the formula of a substance, using the RFM.
%X = (mass of X / RFM) x 100
e.g. percentage of C in C3H8:
% C = (C3/C3H8) x100
= (3x12/(3x12) + 8) x100
= 81.8%