calculate the mass in grams of hydrogen chloride produced when 5.25 L of a molecular hydrogen measured at STP reacts with an excess of molecular chlorine gas

Answer 1

H2 + Cl2 ==> 2HCl
Convert 5.25L Cl2 to mols. L/22.4 = mols.
Convert mols Cl2 t mols HCl using the coefficients in the balanced equation.
Convert mols HCl to grams. g = mols x molar mass.