Which one of the following properties of an element is not variable?

In a reaction,the ferrous $(Fe^{2+})$ ion is oxidized to ferric $(Fe^{3+})$ ion. The equivalent weight of the ion in the above reaction is equal to:

What will be the mass of one atom of ${}^{12}C$?

An element,$X$ has the following isotopic composition:
$^{200}X: 90\%$,$^{199}X: 8.0\%$,$^{202}X: 2.0\%$
The weighted average atomic mass of the naturally occurring element $X$ is closest to......$amu$.

$A$ gas has the formula $(CO)_x$. Its vapour density is $70$. What is the value of $x$?

Mass of one molecule of oxygen in $amu$ and in $gram$ respectively is