Crystals of hydrated cobalt(II) chloride, CoCl2⋅xH2O\text{CoCl}_2 \cdot x\text{H}_2\text{O}CoCl2⋅xH2O, 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 cobalt(II) chloride crystals are heated, they decompose according to the following equation:
CoCl2⋅xH2O→CoCl2+xH2O \text{CoCl}_2 \cdot x\text{H}_2\text{O} \to \text{CoCl}_2 + x\text{H}_2\text{O} CoCl2⋅xH2O→CoCl2+xH2OThe following masses were recorded:
Calculate the mass of CoCl2\text{CoCl}_2CoCl2 formed after heating to constant mass.
Calculate the mass of water lost from the crystals.
The relative formula mass of CoCl2\text{CoCl}_2CoCl2 is 130. The relative formula mass of water is 18. Use these values and your answers to (a)(i) and (a)(ii) to calculate the value of x x\,x in the formula CoCl2⋅xH2O\text{CoCl}_2 \cdot x\text{H}_2\text{O}CoCl2⋅xH2O.
Why is it necessary to heat the crystals to constant mass?
Describe a chemical test to show that the liquid lost was indeed water, including the expected observation.
80 exam-style questions on Edexcel IGCSE Chemistry Chemical formulae, equations and calculations. Each one has a worked solution and a mark scheme showing where the marks go.