Question

A hydrated form of copper sulfate (CuSO4 x H2O) is heated to drive off all the water. If we start with 8.18 g of hydrated salt and have 7.35 g of anhydrous CuSO4 after heating, find the number of water molecules associated with each CuSO4 formula unit.

Answer 1

Consider dehydration reaction of CuSO4. xH2O

CuSO4. x H2O ----------> CuSO4 + x H2O

Molar mass of CuSO4 = 159.609 g/mol

Molar mass of H2O = 18 g/mol

Molar mass of CuSO4. x H2O = (159.609 + x * 18)g/mol

Number of moles of CuSO4. x H2O

= 8.18 g / (159.609 +18x) g/mol = 8.18 mol /(159.609 + 18x)

Number of moles of CuSO4 = 7.35 g / 159.609 g/mol = 0.046 mol

From balanced reaction,

0.046 mol = 8.18 mol / ( 159.609 + 18 x)

8.18 mol = 0.046 mol * (159.609 + 18 x)

8.18 = 7.342 + 0.828 x

0.828 x =8.18 - 7.342 = 0.838

x = 0.838 / 0.828 = 1

Number of water molecules associated with each CuSO4 formula unit = x = 1