stoichiometry(correct and quick answer quick rate)?

Question

13.4 g of a sample of a salt Na2S04. x H2O was found to contain 6.3 g of water. Determine the number of molecules of water of crystallisation.

Answer 1

First, you need to find out how many moles of each substance you have. 13.4 grams is the total mass, and 6.3 grams is water, so 13.4 - 6.3 = 7.1 grams is Na2SO4.

The molar mass of water is 18.02 grams per mole. The molar mass of sodium sulfate is 142.04 grams per mole. Therefore, the sample contains:

6.3 g / (18.02 g/mol) = 0.35 mol water
and
7.1 g / (142.04 g/mol) = 0.050 mol sodium sulfate

The hydration is going to be a small whole number ratio, so we're looking for a whole number ratio between the two numbers of moles.

0.35 / 0.050 = 7

So, the formula is
Na2SO4 ▪ 7 H2O