Question

Hydrated sodium carbonate weighting 4 g, contains 2.52 g of water of crystallisation. The formula of hydrated salt

Answer 1

In order to solve this you have to realize that chemical formulas are mole ratios, but you are given masses.

 

Since you know that 3.00g of FeSO4•nH2O produce 1.36g of water, it stands to reason that the compound contains 1.64g of "dry" FeSO4 (no mass is gained or lost in a reaction).

 

So your "parts" ratio, in grams, is:

 

1.64g FeSO4 : 1.36g H2O

 

The get a formula, convert your masses to moles:

 

1.64g FeSO4 x (1mol/151.91g) = 0.01079mol

 

1.36g H2O x (1mol/18.016g) = 0.07554mol

 

Now you can get the ratio of H2O to FeSO4

 

0.007554mol / 0.01079 mol ≈ 7

 

So your formula is FeSO4•7H2O

 

You should try to do this similar problem to check your understanding: "3.00g of CuSO4•nH2O produces 1.92g of CuSO4 upon dehydration. What is the original formula of the compound?"

 

Hints and the answer are below:

 

1) get the mass of water lost by difference

 

2) convert the masses to moles

 

3) do a mole ratio (water on top)

 

Answer: the ratio is 5, so the formula is CuSO4•5H2O