Answer
a) Composition of a compound is based on the the amount of the both atoms, not on the previous mixture.
b)
The volume of $2HCl$ will be equal to the volume of either the $H_{2}$ reacted or $Cl_{2}$
Work Step by Step
b)
Avogadro's hypothesis is discusses that the ratio of volume are consistently equal to the ratio of molecules provided that the pressure and temperature are the same.
Therefore, since the reaction is $H_{2} (g) + Cl_{2} (g)$ -> $2HCl$
The volume of $2HCl$ will be equal to the volume of either the $H_{2}$ reacted or $Cl_{2}$
