Can someone solve this problem? (chemistry)

Question

4.2 grams of metallic bonding X-Mg (where X is a trivalent metal) reacts with HCl and 3.99 liters of H are released. Knowing that the percentage of Mg in the bond is 91.4% and N(number of neutrons) of x is 14 find the Z(number of protons) of X

Answer 1

3.99 liters of hydrogen will equate to different numbers of moles depending on the temperature and pressure. What is the temperature and pressure? In the absence of this info, considering X is a trivalent metal and has 14 neutrons, the likely answer to what Z is would be Z = 13, making X-Mg an aluminum alloy of magnesium.

I’m sure that’s not the way this is supposed to be solved, but in the absence of temperature and pressure, it’s difficult to know moles of H₂ and consequently moles of other elements.