A mixture of aluminium and zinc weighing 1.67 grams was completely dissolved in acid and 1.69 litres of hydrogen gas was evolved. The hydrogen gas was measured at STP conditions. What was the mass percent of aluminium in original mixture?

Using the molar volume of a gas at STP, the evolved hydrogen is 1.69 divided by 22.4, which is 0.0754 moles. Let the mass of aluminium be y grams, making the mass of zinc equal to (1.67 - y) grams. Setting up the hydrogen evolution equation based on reaction stoichiometry gives y/18 + (1.67 - y)/65.4 = 0.0754. Solving this equation results in the mass of aluminium being 1.25 grams, which gives a mass percentage of 1.25 divided by 1.67, equal to 75.

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