The oxide,which is best soluble in $H_2O$ is

Which of the following salts can accommodate the largest number of $H_2O$ molecules per molecule in their halide hydrates?

When the ash obtained by burning a metal in air is moistened,it gives the smell of $NH_3$. Which metal is it?

The correct sequence of thermal stability of the following carbonates is:

$A$ metal $M$ readily forms its sulphate $MSO_4$ which is water-soluble. It forms its oxide $MO$ which becomes inert on heating. It forms its insoluble hydroxide $M(OH)_2$ which is soluble in $NaOH$ solution. Then $M$ is

Assertion : Both $Be$ and $Al$ can form complexes such as $BeF_4^{2-}$ and $AlF_6^{3-}$ respectively,$BeF_6^{3-}$ is not formed.
Reason : In case of $Be$,no vacant $d-$ orbitals are present in its outermost shell.