how many grams of magnesium are required to liberate 250 cm3 of Hydrogen gas from HCL

Answer 1

Mg + 2HCl ==> H2 + MgCl2
You want 250 cc H2. I assume that's at STP. mols H2 = 250 cc x (1 mol/22,400cc) = ?
Convert mols H2 to mols Mg using the coefficients in the balanced equation.
?mol H2 x (1 mol Mg/1 mol H2) =- ? mol H2 x (1/1) = ? mols Mg.
Now convert mols Mg to grams. g = mols x molar mass.