If 0.525 g of Fe2O3.xH2O are heated and 0.389 g of Fe2O3 are obtained

Question

If 0.525 g of Fe2O3.xH2O are heated and 0.389 g of Fe2O3 are obtained, what is the value of x?

Your answer should have three significant figures. In this case x does not have to be a whole number.

Answer 1

Heating will remove the water of hydration so the difference in mass before and after heating gives y the mass of H₂O. 

0.525 g - 0.389 = 0.136 g H2O

Convert to moles:

0.136 g/18g/mol = 0.00756 mol

Find moles Fe₂O₃ in 0.389 g

0.389g/159.7g/mol = 0.00244 mol 

What’s the mole ratio of salt to water?  It is .00756/.00244 = 3.0984

meaning each mole of salt has 3.0984 moles H₂O. 

Value of x is 3.10 to three sig figs