Balanced equation: Mg+2HCl=MgCl2 + H2
Showing that if Mg is not a limiting factor then 2 moles of HCl on complete reaction liberate 1 mole of Hydrogen
therefore:
1 mole of HCl=35.5g, 40g=x
X= 40/35.5= 1.127mole
2 moles of HCl = 1 moles of Hydrogen
1.127=X
X=1.127/2 = 0.56
the theoretical yield of hydrogen