What relative molecular mass is
The definition is the one from section 7.1 — read "of a molecule" where it said "of an atom".
Relative molecular mass (Mr)
The number showing how many times the mass of a molecule of an element or a compound goes into 1/12 of the mass of an atom of the 12C isotope is the relative molecular mass of that element or molecule.
Mr = mass of one molecule ÷ (1/12 × mass of one 12C atom)
Example 1 — carbon dioxide
Problem
The real mass of a molecule of carbon dioxide (CO2) is 7.31 × 10-23 g. The real mass of a carbon atom is 1.99 × 10-23 g. Find the relative molecular mass of CO2.
Solution
Mr = mass of one CO2 molecule ÷ (1/12 × mass of one 12C atom)
= (7.31 × 10-23 g) ÷ (1/12 × 1.99 × 10-23 g)
= 44
Example 2 — water
Problem
The mass of a molecule of water (H2O) is 2.99 × 10-23 g. The atomic mass unit is 1.67 × 10-24 g. Find the relative molecular mass of water.
Solution
Mr = mass of one water molecule ÷ atomic mass unit
= (2.99 × 10-23 g) ÷ (1.67 × 10-24 g)
= 18
This has no unit either
Like the relative atomic mass, the relative molecular mass carries no unit. There is a mass on the top and a mass on the bottom, so the grams cancel.