calculation

Chemical formula of hydrated sodium carbonate is Na2CO3·10H2O.

Na2CO3·10H2O (s) → Na2CO3 (s) + 10H2O(l)

Mole ratio of Na2CO3·10H2O : H2O = 1:10
Molar mass of Na2CO3·10H2O
= 23.1 X2 + 12.0 + 16.0 X 3 + 10 X (1.0 X 2 + 16.0)
= 286.2 g/mol
Number of mole of Na2CO3·10H2O
= mass / molar mass
= 10 / 286.2
= 0.0349406 mol

So, number of mole of water vapour formed
= 0.0349406 X 10
= 0.349406 mol
Molar mass of water = 1.0 X 2 + 16.0 = 18.0 g/mol
Therefore, mass of water vapour
= molar mass X no. of mole
= 18.0 X 0.349406
= 6.29 g

This is the calculated amount, however, some of the reactant may not react, and the real figure may be smaller than this figure. So the answer is C. 5.12 g

(Relative atomic mass: H: 1.0 C: 12.0 O: 16.0 Na: 23.1)

2007-02-20 22:02:30 補充：
係呀，一定係10個水 (for Na2CO3)。因為一個Na2CO3可以吸十個水。呢個crystallization of water唔係喺會考課程入面，所以我都唔知點解，唯有輕輕背一背。P.S. CuSO4 就有5個水，即係CuSO4‧5H2O