First work out the relative mass of Na₂S₂O₅:

Mass = (2 × 23) + (2 × 32) + (5 ×16) = 46 + 64 + 80 = 190

Percentage composition of sulphur = (64 ¸190) × 100 = 33.7 %

 Equation:

Na₂S₂O₅  ® Na₂SO₃ + SO₂

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