Which of the following pairs of substances cannot exist together in a solution?

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,$pH = pK_a$.

The ionization constant of dimethylamine is $5.4 \times 10^{-4}$. Calculate its degree of ionization in its $0.02 \ M$ solution. What percentage of dimethylamine is ionized if the solution is also $0.1 \ M$ in $NaOH$?

Arrange the following $1 \ L$ aqueous solutions of equal concentration in increasing order of their $pH$:
$CH_3COONa, KI, NH_4Cl, HNO_3$

At $25^{\circ} C$, the molar conductance of $0.007 \ M$ hydrofluoric acid is $150 \ \text{mho} \ cm^{2} \ mol^{-1}$ and its $\Lambda_{m}^{\circ} = 500 \ \text{mho} \ cm^{2} \ mol^{-1}$. The value of the dissociation constant of the acid at the given concentration at $25^{\circ} C$ is

The first and second dissociation constants of a diprotic acid ${H_2}A$ are $1.0 \times 10^{-5}$ and $5.0 \times 10^{-10}$ respectively. The overall dissociation constant of the acid ${H_2}A$ is .......