Calculate the theoretical volume of gas you would expect to be evolved

Question

from your experiment if X moles of magnesium were reacted with an
excess of HCl at standard conditions (STP).

Answer 1

Mg + 2HCl ==> MgCl2 + H2

mols Mg = X
mols H2 produced = X (look at the coefficients; 1 mol H2 produced for 1 mol Mg used)
Volume = mols*22.4L/mol
volume 22.4X liters.