The heaviest atom amongst the following is

At $STP$,if $5.6 \ L$ of a gas weighs $7.5 \ g$,then identify the gas.

In a metal oxide,there is $20\%$ oxygen by weight. Its equivalent weight is

The vapor density of a metal chloride is $77$. If the equivalent weight of the metal is $3$,what is its atomic weight?

$A$ mixture of $1.65 \times 10^{21}$ molecules of $X$ and $1.85 \times 10^{21}$ molecules of $Y$ weighs $0.688 \ g$. If the molecular mass of $Y$ is $187 \ g/mol$,what is the molecular mass of $X$?

What are the equivalent weights of $H_3PO_4$ in the following reactions respectively?
$H_3PO_4 + OH^{-} \rightarrow H_2PO_4^{-} + H_2O$
$H_3PO_4 + 2OH^{-} \rightarrow HPO_4^{2-} + 2H_2O$
$H_3PO_4 + 3OH^{-} \rightarrow PO_4^{3-} + 3H_2O$