Question

Washing soda has formula Na₂CO₃.10H₂O. What mass of anhydrous sodium carbonate is left when all the water of crystallization is expelled by heating 57.2 g of washing soda?

Answer 1

\[\ce{Na2CO3.10H2O ->[Heat] Na2CO3 + 10H2O}\]

Molecular weight of Na₂CO₃.10H₂O = 23 × 2 + 12 + 16 × 3 + 10[1 × 2 + 16]

= 46 + 12 + 48 + 10[18]

= 106 + 180

= 286

Molecular weight of Na₂CO₃ = 106

∵ 286 g Na₂CO₃.10H₂O left 106 g Na₂CO₃

∴ 57.2 g Na₂CO₃.10H₂O left Na₂CO₃ = `(57.2 xx 106)/286`

= 21.2 g Na₂CO₃

The mass of anhydrous sodium carbonate is 21.2 g.