${\mathrm{H}}_2+{\mathrm{Cl}}_2\to 2\mathrm{HCl}$ . Based on this reaction to prepare one mole of HCl the number of moles of ${\mathrm{Cl}}_2$  required is

${\mathrm{H}}_2+{\mathrm{Cl}}_2\to 2\mathrm{HCl}$ In the given equation 1 mole of ${\mathrm{Cl}}_2$ reacts with 1 mole of ${\mathrm{H}}_2$ to produce 2 moles of HCl 

Hence 1 mole of HCl will be produced from half mole of chlorine and half mole of Hydrogen gases.