The reaction involved in the production of hydrogen (H2) gas from HCl is not given. It is assumed that an atom of hydrogen is released from each molecule of HCl. To produce a molecule of hydrogen, two molecules of HCl are required.
The amount of hydrogen (H2) gas formed is 1.26 L. Using the ideal gas law, PV = nRT, n = PV/RT.
At STP, the temperature T = 273.15 K and the pressure P = 100 kPa. The gas constant R = 8.314472 L kPa/K*mol
Substituting these values n = 100*1.26/273.15*8.314472 = 0.055 moles.
To produce 0.055 moles of hydrogen gas, twice the amount of HCl is required. The number of moles of HCl required is 0.11 mole. The molar mass of HCl is 36.46 g/mol. 0.11 moles of HCl has a mass of 4.045 g
4.045 g of HCl is required to produce 1.26 L of H2 at STP.