Dulong and Petit's law is valid only for

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

The atomic weight of $Ne$ is $20.2$. It is a mixture of $Ne^{20}$ and $Ne^{22}$. What is the relative abundance of the heavier isotope (in $\%$)?

$A$ metal chloride contains $49.5\%$ chlorine. The specific heat of the metal is $0.064 \ cal \ g^{-1} \ ^\circ C^{-1}$. What is the exact atomic weight of the metal?

The equivalent weight of a metal is $9$ and vapour density of its chloride is $59.25$. The atomic weight of the metal is:

The equivalent weight of $Zn(OH)_2$ in the following reaction is equal to its: $Zn(OH)_2 + HNO_3 \to Zn(OH)(NO_3) + H_2O$