A 15.00g of hydrated salt, Na2SO4.xH2O was found to contain 7.05g water. Determine empirical formula of salt.?

Question

A 15.00g of hydrated salt, Na2SO4.xH2O was found to contain 7.05g water. Determine empirical formula of salt.?

Answer 1

The 7.05 grams of water means that almost 50% of the mass of the hydrate is water.  Determine the molar mass of the Na2SO4.  The molar mass of water is 18grams per mole.  So divide the 18 into the mass of Na2SO4 and round off to the nearest integer and that will be the number of molecules of water.

Answer 2

molar mass of Na2SO4 = 2*23 + 32 + 4*16 = 142

molar mass of H2O = 2*1 + 16 =

18x/142 = 7.05/(15-7.05)

x = (7.05/7.95)(142/18) = 7

x = 7

The empirical formula of this salt is Na2SO4.7H2O.