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