How many liters of hydrogen needs to be reacted with chlorine to produce 21g of hydrochloric acid, at STP?
H2 + Cl2 ==> 2HCl
mols HCl = 21/molar mass HCl.
mols H2 = 1/2 that (from the coefficients in the balanced equation).
mols H2 at STP = L/22.4; therefore
L H2 = mols H2 x 22.4 L/mol = ?