Question

A white powder is known to be a mixture of magnesium oxide and aluminium oxide.
100cm3
of 2 moldm–3 NaOH(aq) is just sufficient to cause the aluminium oxide in x grams of the
mixture to dissolve.
The reaction occurring is Al 2O3 + 2OH–
+ 3H2O → 2Al(OH)4
–
.
800cm3
of 2 moldm–3 HCl(aq) is just sufficient to cause all of the oxide in x grams of the mixture
to dissolve.
The reactions occurring are Al 2O3 + 6H+
→ 2Al
3+ + 3H2O
and MgO + 2H+
→ Mg2+ + H2O.
How many moles of each oxide are present in x grams of the mixture?

Answer 1

The number of moles of Al2O3 and MgO present in x gram of the mixture is equal to 0.1 and 0.5 respectively.

• Number of moles of NaOH required to dissolve Al2O3 is equal to 0.2(0.1×2).
• Two moles of NaOH dissolve one mole of Al2O3, hence 0.2 moles of NaOH will dissolve 0.1 moles of Al2O3.
• Therefore, the moles of Al2O3 is equal to 0.1.
• Total moles of HCl used is equal to 1.6 (0.8×2).
• One mole of Al2O3 uses 6 moles of HCl, so 0.1 moles of Al2O3 will use 0.6 moles of HCl, hence HCl left is equal to one mole.
• The left over HCl will be used by MgO, since two moles of HCl dissolves one mole of MgO, One mole of HCl will dissolve 0.5 moles of MgO.
• Therefore moles of MgO present is equal to 0.5