Washing Soda (Chemistry)

Question

Washing soda is a hydrate of sodium carbonate. Its formula is Na2CO3. x H2O. A 2.714 g Sample of washing soda is heated until a constant mass of 1.006 g of Na2CO3 is reached. What is x ?

Answer 1

get moles:

1.006 g Na2CO3 @ 105.99 g/mol = 0.0095 moles Na2CO3

1.708 g of X H2O @ 18.02 g/mole = 0.95 moles H2O

ratio the moles:

moles Na2CO3 : 0.0095 / 0.095 = 1 mole Na2CO3

moles X H2O:   0.95 / 0.095 = 10 moles H2O

Yoiure answer is : Na2CO3.10H2O

Answer 2

Perhaps the problem that caused you to ask this isn't so much the calculation, but realizing that heating the compound will cause the water in the hydrate to leave.  Heating to constant mass means that all the water that will leave has been made to leave, then the problem states that there is just sodium carbonate (no hydrate) at the end.  So there is some water in it to start, and it is all driven off at the end.

Then you can calculate:

1.006 g sodium carbonate, MW 104 remains behind.

1.708 g of water, MW 18, was driven off.

9.6 millimoles of sodium carbonate therefore left behind, and

95 millimoles of water were driven off.

So the ratio is 95/9.6 = 9.9 and since molecules come in integer values, that means X in the formula is 10