Hi!
For a, since not all the magnesium will produce hydrogen gas, AND knowing that the formula used to calculate the molar volume is
Vm = V/n,
we know that this will lower the value of Vstp. In fact, since the number of mols is found using the mass, and the mass of magnesium is bigger than it needs to be, the volume of hydrogen will be divided by a number that is bigger than the mass of magnesium needed to produce the corresponding volume.
As for c, if some gas escaped, the value of V will also decrease because in the formula, we will be dividing a volume that is smaller than what was actually produced.
For b, I may need more precision on the lab to properly answer your question.
Hope I helped!
