A table of molecules

Put the adding-up method to work on several molecules. Read the middle column of each row and see whether you can produce the right-hand one yourself.

Species	Molecular formula	Relative molecular mass
Hydrogen	H₂	2 × 1 = 2
Nitrogen	N₂	2 × 14 = 28
Oxygen	O₂	2 × 16 = 32
Carbon dioxide	CO₂	12 + (2 × 16) = 44
Glucose	C₆H₁₂O₆	(6 × 12) + (12 × 1) + (6 × 16) = 180

Three more, for you to try

Exercise 01

Calculate the relative molecular masses of the following compounds. Try them before looking at the answers.

Ammonia (NH₃) — A_r: H = 1, N = 14
Sulphuric acid (H₂SO₄) — A_r: H = 1, O = 16, S = 32
Sucrose (C₁₂H₂₂O₁₁) — A_r: H = 1, C = 12, O = 16

Answers

NH₃ = 14 + (3 × 1) = 17

H₂SO₄ = (2 × 1) + 32 + (4 × 16) = 98

C₁₂H₂₂O₁₁ = (12 × 12) + (22 × 1) + (11 × 16) = 342

Sucrose means adding up 45 atoms, and that is where the big errors happen. Writing out the three kinds of atom separately and adding at the end is the safe way.