What Is the Percent Composition of Magnesium and Aluminum in the Alloy?

[johndoe]

A metal alloy consists of magnesium and aluminum.When a 1.00g sample of the alloy is reacted with excess HCl, 0.0989g of hydrogen gas forms. Calculate the percent composition of the alloy.

Ok I got the following balanced equations :

Mg + 2HCl -> MgCl2 + H2
2Al + 6Hcl -> 2AlCl3 + 3H2

Can I say 1 mole of Mg reacts with 1 Mole of H2, with H2 being 1/4 of 0.0989g and
2 moles of Al reacts with 3 moles of H2, with H2 being 3/4 of 0.0989g?

 

[Borek]

No, you can't. That will happen only if you will know you have mixture of 1 mole of Mg and 2 moles of Al, you don't know neithe rhow many moles of both nor in what ratio you have.

Write two equations - one for sum of masses of metals, one for sum of numbers of moles of metals. The latter will use moles of hydrogen produced and stoichiometry of the reactions provided.