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Question 51

Crystals of hydrated magnesium sulfate, MgSO4⋅xH2O\text{MgSO}_4 \cdot x\text{H}_2\text{O}MgSO4​⋅xH2​O, contain water of crystallisation.

A student heated a sample of the hydrated crystals in a crucible to remove the water of crystallisation in order to find the value of xxx. The crucible was heated, allowed to cool, and weighed again. This process was repeated until two consecutive masses were the same (heating to constant mass).

When hydrated magnesium sulfate crystals are heated, they decompose according to the following equation:

MgSO4⋅xH2O→MgSO4+xH2O \text{MgSO}_4 \cdot x\text{H}_2\text{O} \to \text{MgSO}_4 + x\text{H}_2\text{O} MgSO4​⋅xH2​O→MgSO4​+xH2​O

The following masses were recorded:

  • Mass of empty crucible = 12.45 g
  • Mass of crucible +MgSO4⋅xH2O=18.60 g+ \text{MgSO}_4 \cdot x\text{H}_2\text{O} = 18.60 \text{ g}+MgSO4​⋅xH2​O=18.60 g
  • Mass of crucible and MgSO4\text{MgSO}_4MgSO4​ after heating to constant mass = 15.45 g
a.

Calculate the mass of MgSO4\text{MgSO}_4MgSO4​ formed after heating to constant mass.

[1]
b.

Calculate the mass of water lost from the crystals.

[1]
c.

The relative formula mass of MgSO4\text{MgSO}_4MgSO4​ is 120. The relative formula mass of water is 18. Use these values and your answers to (i) and (ii) to calculate the value of x x\,x in the formula MgSO4⋅xH2O\text{MgSO}_4 \cdot x\text{H}_2\text{O}MgSO4​⋅xH2​O.

[3]
d.

Why is it necessary to heat the crystals to constant mass?

[1]
e.

Describe a chemical test to show that the liquid lost was indeed water, including the expected observation.

[2]

Chemical formulae, equations and calculations Questions

  1. IGCSE
  2. /Chemistry
  3. /Chemical formulae, equations and calculations