If the concentration of $OH^{-}$ ions is $10^{-7} \text{ gram ions/litre}$,what is the number of $OH^{-}$ ions in one litre of water?

What is the $[OH^-]$ in the final solution prepared by mixing $20.0 \ mL$ of $0.050 \ M \ HCl$ with $30.0 \ mL$ of $0.10 \ M \ Ba(OH)_2$?

On decreasing the $pH$ from $7$ to $2$,the solubility of a sparingly soluble salt $(MX)$ of a weak acid $(HX)$ increased from $10^{-4} \ mol \ L^{-1}$ to $10^{-3} \ mol \ L^{-1}$. The $pK_{a}$ of $HX$ is:

$A$ solution of $0.1 \ M$ weak base $(B)$ is titrated with $0.1 \ M$ of a strong acid $(HA)$. The variation of $pH$ of the solution with the volume of $HA$ added is shown in the figure below. What is the $pK_{b}$ of the base? The neutralization reaction is given by $B + HA \rightarrow BH^{+} + A^{-}$.

For the reaction in aqueous solution $Zn^{2+} + X^{-} \rightleftharpoons ZnX^{+}$,the $K_{eq}$ is greatest when $X$ is

$A$ $20 \ mL$ sample of a $0.2 \ M$ solution of the weak diprotic acid $H_2A$ is titrated with $0.25 \ M \ NaOH$. Molarity of the solution at the second equivalence point is: