Question

A hydrated form of copper sulfate ( CuSO 4 ⋅ x H 2 O ) is heated to drive off all of the water. If there is initially 7.74 g of hydrated salt and there is 4.95 g of anhydrous CuSO 4 after heating, find the number of water molecules associated with each CuSO 4 formula unit.

Answer 1

Answer: 5 water molecules

Step-by-step explanation:

Decomposition of hydrated copper sulphate is given by:

`CuSO_4.xH_2O\rightarrow CuSO_4+xH_2O`

Molar mass of
`CuSO_4` = 159.6 g/mol

According to stoichiometry:

(159.6+18x) g of
`CuSO_4.xH_2O` decomposes to give = 159.6g of
`CuSO_4`

Thus 7.74 g of
`CuSO_4.xH_2O` decomposes to give=
`(159.6)/(159.6+18x)* 7.74` g

But it is given 7.74 g of a sample of hydrated salt was found to contain 4.95 g of anhydrous salt.

Thus we can equate the two equations:

`(159.6)/(159.6+18x)* 7.74=4.95`

x=5

Thus there are 5 water molecules associated with each
`CuSO_4` formula unit.