The different volumes of hydrogen produced, and therefore the different numbers of moles, can be explained by the stoichiometry of the reactions. Here are the balanced equation for the reactions:
`Zn + 2 HCl -> ZnCl_2 + H_2`
`Mg + 2 HCl -> MgCl_2 + H_2`
`2 Al + 6 HCl -> 2 AlCl_3 + 3 H_2`
The same number of moles of each metal was used. The coefficients from the balanced equations can be used to find out how many moles of hydrogen gas is produced from a given number of moles of metal. The ratio of H2 produced to metal consumed is the coefficient of H2 over the coefficient of the metal. Since we don't know how many moles were used, I'll use one mole for each metal.
Zinc: (1 mole Zn) x (1 mole H2/1 mole Zn) = 1 mole H2
Magnesium: (1 mole Mg) x (1 mole H2/1 mole Mg) = 1 mol H2
Aluminum: (1 mole Al) x (3 moles H2/2 moles Al) = 1.5 moles H2
The reaction of aluminum with excess HCl produces 1.5 moles of hydrogen gas per mole of aluminum. Magnesium and Zinc reacting with HCl both produce 1 mole of hydrogen gas for every one mole of metal.
Since moles of gas are proportional to volume at the same temperature and pressure, we would expect 1.5 times the volume of hydrogen from the aluminum reaction than from the other two, as the graph shows.