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⋅xH2O.
She uses this method:
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⋅xH2O→CoCl2+xH2OThese are the student's results:
State why it is necessary to heat the crystals to constant mass.
Calculate the mass of CoCl2\text{CoCl}_2CoCl2 formed after heating to constant mass.
Calculate the mass of water lost from the crystals.
Calculate the value for x x\,x in the formula CoCl2⋅xH2O\text{CoCl}_2 \cdot x\text{H}_2\text{O}CoCl2⋅xH2O. 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 H2O=18]
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.
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.