A student heats a 5.00 g sample of hydrated magnesium sulfate, MgSO4⋅7H2O\text{MgSO}_4\cdot 7\text{H}_2\text{O}MgSO4⋅7H2O, strongly in a crucible to drive off the water of crystallisation and form anhydrous magnesium sulfate, MgSO4\text{MgSO}_4MgSO4:
MgSO4⋅7H2O(s)→MgSO4(s)+7H2O(g) \text{MgSO}_4\cdot 7\text{H}_2\text{O(s)} \rightarrow \text{MgSO}_4\text{(s)} + 7\text{H}_2\text{O(g)} MgSO4⋅7H2O(s)→MgSO4(s)+7H2O(g)The mass of the solid residue remaining in the crucible is measured to be 2.75 g, which is higher than the calculated theoretical mass of anhydrous magnesium sulfate (2.44 g).
Which of the following is a likely reason for this higher actual mass?
The crucible was weighed while still hot, generating upward convection currents of air.
Some of the solid spat out of the crucible during the heating process.
The dehydration of the hydrated magnesium sulfate was incomplete.
The anhydrous magnesium sulfate underwent further thermal decomposition to form magnesium oxide and gaseous sulfur trioxide.
258 exam-style questions on Edexcel GCSE Chemistry Calculations involving masses. Each one has a worked solution and a mark scheme showing where the marks go.