Different proportions of oxygen in the various oxides of nitrogen prove the

The law of definite proportions is not applicable to nitrogen oxides because

What weight of $BaCl_2$ is required to react with $24.4 \ g$ of sodium sulfate to produce $46.6 \ g$ of barium sulfate and $23.4 \ g$ of sodium chloride (in $g$)?

Which of the following pairs of compounds cannot demonstrate the law of multiple proportions?

When $2.16 \ g$ of metallic copper is treated with nitric acid,the weight of copper oxide obtained is $2.70 \ g$. In a second experiment,$1.15 \ g$ of copper oxide on reduction gives $0.92 \ g$ of copper. These results illustrate the law of:

Define the Law of Multiple Proportions.