Question

: Epsom salts is a hydrated ionic compound with the following formula: MgSO4 • xH2O. A 4.93 g sample of Epsom salts was heated to drive off the water of hydration. The mass of the sample after complete dehydration was 2.41 g. Find the number of waters of hydration x in the salts.

Answer 1

The mass of Epsom was 4.93 g and its reduced mass after heaing was 2.41 g.

Hence, the water weight is 4.93 g - 2.41 g = 2.52 g

Now find the number of moles of MgSO₄ and water in orignal sample i. e. the MgSO₄ weight found after dehydration, 2.41 g

Moles = weight of sample/ Molecular weight

         = 2.41 g/120 gmo^l-1 = 0.0201 mol MgSO₄

Similarly,

number of moles of water in orignal sampleis given as,

moles of water = 2.52 g / 18 gmol^-1 = 0.14 mol water

Now, to find the ratio between MgSO4 and water we have to divide obtained mole with the smallest number i. e.

for MgSO₄ = 0.0201/0.0201 = 1

for water = 0.14/0.0201 = 7

Hence we can see that ratio of MgSO₄ to water is 1:7

Therefore x = 7 = number of hydration in salts