In this type of problem, one of the reactants is usually in excess. To determine the reactant, we need to determine the number of moles of each the reactants. The first step is to determine mass of one mole of P4.
Mass = 4 * 31 = 124 grams
For P4, n = 20.0 ÷ 124
C 0.16 mole.
For HCl, n = 5 * 7.225 = 36.125
According to the coefficients in this equation, one mole of phosphorous reacts with twelve moles of hydrochloric acid. Since the number of moles of hydrochloric acid is much greater than twelve times the number of moles of phosphoric acid, there is excess hydrochloric acid. According to the coefficients in this equation, one mole of phosphorous will produce six moles of hydrochloric acid.
n = 6 * 20.0 ÷ 124 = 6 ÷ 6.2
This is approximately 0.97 mole. At STP, the volume of one mole of a gas is 22.4 liters.
V = 22.4 * 6 ÷ 6.2
This is approximately 21.7 moles. Now you have two answers that are the same. I hope this is helpful for you.