Which substance is used to produce very low temperatures by magnetic effects?

Among $Co^{3+}$,$Ti^{2+}$,$V^{2+}$ and $Cr^{2+}$ ions,one if used as a reagent cannot liberate $H_{2}$ from dilute mineral acid solution,its spin-only magnetic moment in gaseous state is $.....$ $B.M.$ (Nearest integer)

Boiling $CuCl_2$ with $Cu$ in concentrated $HCl$ gives:

On heating a mixture of common salt and $K_{2}Cr_{2}O_{7}$ with concentrated $H_{2}SO_{4}$, a gas is evolved. The formula of the gas evolved and the oxidation state of the central metal atom in the gas, respectively, are:

Which element forms the maximum number of stable oxides?

$KMnO_4$ is the oxo-salt of the acid of which of the following oxides?