How many litres of hydrogen at 0 celsius and 1400 mm Hg are produced if 15g of magnesium reacts with sulphuric acid

I assume you mean an (excess) of H2SO4.

Mg + H2SO4 ==> MgSO4 + H2
mols Mg = g/atomic mass = 15/24.3 = ?
Convert mols Mg to mols H2 produced. You know from the equation that 1 mol Mg will produce 1 mol H2 gas.
Then use PV = nRT to calculate volume of H2 in liters.
Remember that 1400 mm Hg pressure = 1400/760 = ? atmospheres P.
Remember 0 oC = 273 K
You have n = mols Mg
Post your work if you get stuck.