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Chemical formulae, equations and calculations

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Question 9

A student wants to find the value of x x\,x in hydrated cobalt(II) chloride, CoCl2⋅xH2O\text{CoCl}_2 \cdot x\text{H}_2\text{O}CoCl2​⋅xH2​O.

She uses this method:

  • weigh an empty crucible to find its mass
  • place a sample of hydrated cobalt(II) chloride crystals into the crucible and reweigh
  • heat the crucible gently
  • allow the crucible to cool and reweigh
  • repeat the process until the mass no longer changes (heating to constant mass)

The crystals decompose according to this equation:

CoCl2⋅xH2O→CoCl2+xH2O \text{CoCl}_2 \cdot x\text{H}_2\text{O} \rightarrow \text{CoCl}_2 + x\text{H}_2\text{O} CoCl2​⋅xH2​O→CoCl2​+xH2​O

These are the student's results:

  • Mass of empty crucible: 15.35 g
  • Mass of crucible and CoCl2⋅xH2O\text{CoCl}_2 \cdot x\text{H}_2\text{O}CoCl2​⋅xH2​O: 20.11 g
  • Mass of crucible and contents after heating to constant mass: 17.95 g
a.

State why it is necessary to heat the crystals to constant mass.

[1]
b.

Calculate the mass of CoCl2\text{CoCl}_2CoCl2​ formed after heating to constant mass.

[1]
c.

Calculate the mass of water lost from the crystals.

[1]
d.

Calculate the value for x x\,x in the formula CoCl2⋅xH2O\text{CoCl}_2 \cdot x\text{H}_2\text{O}CoCl2​⋅xH2​O. Give your answer to the nearest whole number. [Mr of CoCl2=130;Mr of H2O=18][M_r\text{ of }\text{CoCl}_2 = 130; M_r\text{ of }\text{H}_2\text{O} = 18][Mr​ of CoCl2​=130;Mr​ of H2​O=18]

[3]
e.

When water is added back to the anhydrous cobalt(II) chloride, the student observes that the mixture gets warm and changes colour from blue to pink. Explain these observations.

[2]

Chemical formulae, equations and calculations Questions

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