Question

A student has a 2.123 g sample of a hydrated salt. she heats it to drive off all the water. after heating to constant mass, the salt weighs 1.861 g. the anhydrous salt is copper(ii) phosphate. what is the empirical formula of the hydrated salt?

Answer 1

The mass of hydrated salt - 2.123 g
mass of anhydrous salt - 1.861 g
mass that has been reduced is the mass of water that has been heated and lost from the compound thereby making the salt anhydrous.
therefore mass of water lost - 2.123 - 1.861 = 0.262 g
number of moles of water lost - 0.262 g / 18 g/mol = 0.0146 mol 
number of moles of salt - 1.861 g / 380.6 g/mol = 0.00490 mol 
molar ratio of moles of water to moles of salt 
molar ratio = 0.146 mol / 0.00490 mol = 2.98 rounded off to 3
for every 1 mol of salt there are 3 moles of water
therefore empirical formula - Cu₃(PO₄)₂.3H₂O