Pure $N_2$ is obtained by:

How many numbers of $P-OH$ and $P-O-P$ bonds are present in pyrophosphoric acid respectively?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$ : $PH_3$ has lower boiling point than $NH_3$.
Reason $(R)$ : In liquid state $NH_3$ molecules are associated through van der Waals forces,but $PH_3$ molecules are associated through hydrogen bonding.
In the light of the above statements,choose the most appropriate answer from the options given below:

Of the following hydrides,which has the lowest boiling point?

Which is the correct statement for the given acids?

When hot and concentrated nitric acid reacts with copper,the gas obtained is: