In order to answer this, we will assume that all of the hydrogen reacts with the chlorine and the chlorine is in excess.
The reaction between hydrogen and chlorine for the formation of hydrogen chloride is:
H₂ + Cl₂ → 2HCl
All the species in this reaction are gasses, and for gasses, the ratio of volumes is equivalent to the ratio of moles. From the reaction, we see that each mole of hydrogen produces two moles of hydrogen chloride. So, the volume of HCl produced:
2 x 25.2 = 50.4 Liters
