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CAIE IGCSE Chemistry 3.3.7 Empirical Formulae and Molecular Questions

Determine empirical and molecular formulae from composition, mass data and relative molecular mass.

Syllabus
2026–2028
Course
Chemistry 0620

Exam points

  • calculate empirical formula from percentage composition
  • deduce empirical formula from mass data
  • derive molecular formula from empirical formula and relative mass
  • use combustion and elemental analysis to find formulae
  • determine formulae of hydrated salts and organic compounds
  • relate mole ratios to empirical and molecular formulae

CAIE IGCSE Chemistry 3.3.7 Empirical Formulae and Molecular Questions question 1

[Maximum number: 4]

Hydrated cobalt(II) sulfate, CoSO4⋅xH2O\mathrm{CoSO}_{4} \cdot \mathrm{x} \mathrm{H}_{2} \mathrm{O}, produces water when it is heated.

CoSO4⋅xH2O( s)→CoSO4( s)+xH2O( g)\mathrm{CoSO}_{4} \cdot \mathrm{xH}_{2} \mathrm{O}(\mathrm{~s}) \rightarrow \mathrm{CoSO}_{4}(\mathrm{~s})+\mathrm{xH}_{2} \mathrm{O}(\mathrm{~g})

A student does an experiment to determine the value of x in CoSO4⋅xH2O\mathrm{CoSO}_{4} \cdot \mathbf{x} \mathrm{H}_{2} \mathrm{O}.
step 1 The student weighs a sample of hydrated cobalt(II) sulfate.
step 2 The student heats the sample of hydrated cobalt(II) sulfate.
step 3 The student weighs the remaining solid after heating.

In an experiment, 1.405 g of CoSO4⋅×H2O\mathrm{CoSO}_{4} \cdot \times \mathrm{H}_{2} \mathrm{O} is heated until all the water is given off.

The mass of CoSO4\mathrm{CoSO}_{4} that remains is 0.775 g .
[Mr:CoSO4,155;H2O,18]\left[M_{\mathrm{r}}: \mathrm{CoSO}_{4}, 155 ; \mathrm{H}_{2} \mathrm{O}, 18\right]
Determine the value of x using the following steps.
- Calculate the number of moles of CoSO4\mathrm{CoSO}_{4} that remains. mol
- Calculate the mass of H2O\mathrm{H}_{2} \mathrm{O} given off. g
- Calculate the number of moles of H2O\mathrm{H}_{2} \mathrm{O} given off. mol
- Determine the value of x.

x=……………………....[4][Total:17]\begin{array}{r} x=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . . . \\ {[4]} \\ {[T o t a l: 17]} \end{array}
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