3.1.2 - Relative Formula Mass

Define relative atomic mass (RAM)

The average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to ¹²C

Define relative formula mass (RFM)

The sum of RAM’s of all atoms in the formula

How do you calculate RFM?

1. Work out how many atoms of each element there are in the chemical formula.

2. Add together the A_r values for all the atoms of each element present.

What is the relative formula mass of:

1. CaF₂

2. C₆H₁₂O₆

1. CaF₂ - (A_r values: Ca = 40, F = 19)

   40 + 19 + 19 = 78

2. CHO - (A values: C = 12, H = 1, O = 16)

   (12 × 6) + (1 × 12) + (16 × 6) = 180

How do you find the percentage mass of an element in a compound?