Sodium metabisulphite is a food preservative (E223).  It works by decomposing to sodium sulphite (Na₂SO₃) and sulphur dioxide when added to water.

Its chemical formula is Na₂S₂O₅.  (Na = 23; S = 32; O = 16)

(a)  What is the percentage composition of sulphur in sodium metabisulphite?

(b) Write a balanced symbol equation to describe its decomposition when added to water.

ANSWER