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

What is ionic equilibrium? Discuss the relation of types of substances and ionic equilibrium in solution.

The pairs of compounds which cannot exist together in aqueous solution are :
$I. NaH_2PO_4$ and $Na_2CO_3$ $II. Na_2CO_3$ and $NaHCO_3$
$III. NaOH$ and $NaH_2PO_4$ $IV. NaHCO_3$ and $NaOH$

If $0.1 \, M, 100 \, mL$ $NaOH$ solution is mixed with $0.05 \, M, 100 \, mL$ $H_2SO_4$ solution,the $pH$ of the resulting solution is:

The ionization constant of $ClCH_{2}COOH$ is $1.35 \times 10^{-3}$. What will be the $pH$ of $0.1 \ M$ acid and its $0.1 \ M$ sodium salt solution?

What is the concentration of $[OH^-]$ in a mixture of $20 \ mL$ of $0.050 \ M \ HCl$ and $30 \ mL$ of $0.10 \ M \ Ba(OH)_2$ (in $M$)?