Which metal sulphide is soluble in excess $NH_3$ solution?

The species present in solution when $CO_2$ is dissolved in water are

When $CO_2$ dissolves in water,the following equilibrium is established:
$CO_2 + 2H_2O \rightleftharpoons H_3O^{+} + HCO_3^-$
For which the equilibrium constant is $3.8 \times 10^{-7}$ and $pH = 6.0$. The ratio of $[HCO_3^-]$ to $[CO_2]$ would be:

Which of the following statement$(s)$ is/are correct.
$A$. The $pH$ of $1 \times 10^{-8} \ M$ aqueous solution of $HCl$ is $8$
$B$. The conjugate base of $H_2PO_4^{\ominus}$ is $HPO_4^{2-}$
$C$. $K_w$ increases with increase in temperature
$D$. When a solution of a weak monoprotic acid is titrated against a strong base,then at the half-neutralization point,$pH = pK_a$.

In a saturated solution of $Mg(OH)_2$,the degree of dissociation of $Mg(OH)_2$ is $\alpha$. Find the concentration $(C)$ of $Mg(OH)_2$ if the concentration of $[OH^{-}]$ is $2$.

The degree of ionization of a solution depends upon: