$CrO_4^{2-}$ (Yellow) changes to $Cr_2O_7^{2-}$ (orange) in $pH = x$ and vice versa in $pH = y$,hence $x$ and $y$ are respectively

If $10\%$ of a $5 \times 10^{-3} \, M \, H_2CO_3$ solution dissociates,then the concentration of $H^{+}$ ions is equal to $......$.

If $20 \, mL$ of $0.25 \, N$ strong acid and $30 \, mL$ of $0.2 \, N$ strong base are mixed,then the resulting solution is

At $25^{\circ}C$, $20.0 \ mL$ of $0.2 \ M$ weak monoprotic acid $HX$ is titrated against $0.2 \ M$ $NaOH$. The $pH$ of the solution $(a)$ at the start of the titration (when $NaOH$ has not been added) and $(b)$ when $10 \ mL$ of $NaOH$ is added respectively, are:
Given: $K_a = 5 \times 10^{-4}, pK_a = 3.3, \alpha << 1$

Which of the following statements are incorrect?

The first and second ionization constants of a weak dibasic acid $H_{2}A$ are $8.1 \times 10^{-8}$ and $1.0 \times 10^{-13}$ respectively. $0.1 \text{ mol}$ of $H_{2}A$ was dissolved in $1 \text{ L}$ of $0.1 \text{ M}$ $HCl$ solution. The concentration of $HA^{-}$ in the resultant solution is: