The fully hydrated form of sodium sulfate is the decahydrate, Na2SO4 • 10H2O. When heated the hydrated salt loses water. How many water molecules are found per formula unit in a partially dehydrated sample of sodium sulfate with a formula mass of 197 amu (i. e. find n for Na2SO4 • nH2O) ?

There are found 3 water molecules per formula unit in a partially dehydrated sample of sodium sulfate with a formula mass of 197 amu.

Explanation:

The fully hydrated form of sodium sulfate is the decahydrate has structural formula of Na2SO4*10H2O

The molar mass of the decahydrate is 323 amu

When heated the hydrated salt loses water.

The molecular mass is now 197 amu

Na2SO4 has a molecular mass of 142.04 amu

1 molecule of water has a molecular mass of 18 amu

Before heating, the mass of Na2SO4*10H2O = 323 amu

After heating, the mass of Na2SO4*nH20 = 197 amu

The mass of nH2O lost = 323-197 = 126 amu

1 * H2O has a molecular mass of 18 amu

n = 126/18 = 7

The mass after heating - the mass of Na2SO4 = The mass of nH2O

197 - 142 = 55

The mass of nH2O / the mass of 1*H2O = n

55 / 18 = 3

This partially dehydrated sample of sodium sulfate is Na2SO4*3H2O

There are found 3 water molecules per formula unit in a partially dehydrated sample of sodium sulfate with a formula mass of 197 amu.