Answer
See explanation
Work Step by Step
a. The composition of a compound depends on the number of atoms of each element of the compound and not on the composition of the mixture from which it was formed. This satisfies the law of definite proportion that one compound will always have elements in same proportion regardless of the process of creation.
b. Avogadro’s hypothesis states that at constant temperature and pressure the ratio of volume of two gases will be same as the ratio of their molecules. Since one molecule of H$_{2}$ reacts with one molecule Cl$_{2}$ to form two molecules of HCl, so for one molecule of H$_{2}$ there will be two molecules of HCl hence the volume of HCl will be twice the volume of H$_{2}$ (or Cl$_{2}$) reacted.
