what is the mass of magnesium which completely reacts with 250cm^3 of 1 mol/dm^3 sulfuric acid?

Mg + H2SO4 = MgSO4 + H2
moles of H2SO4 in the solution = 1 mol/dm^3 * 250 dm^3 / 1000 = 0.25 moles

according to the equation these react with 0.25 mol of Zn
mass of Mg = 0.25 mol * 24.3050 g/mol = 6.1 g of Mg

Balanced equation: H2SO4 + Mg --> MgSO4 + H2(g)
0.250 dm^ X 1 mol/dm^3 = 0.250 mol H2SO4
0.250 mol H2SO4 (1 mol Mg/1 mol H2SO4) = 0.250 mol Mg
Just multiply that by the molar mass of Mg, and you have your answer.