Question

A titration is carried out by slowly adding a solution of $0.125MNaOH\left(aq\right)$ to a flask containing $25.00mL$ of a solution of $H_{2}S{O}_4\left(aq\right)$. The titration continues until the equivalence point is reached, as shown by phenolphthalein indicator.
Which of the following CORRECTLY describes the change in the concentration of sodium ions, $\left[N{a}^{+}\right]$, and the concentration of sulfate ions, $\left[S{O}_4^{2-}\right]$, in the flask during the titration?

• A. $\left[N{a}^{+}\right]$ and $\left[S{O}_4^{2-}\right]$ both increase.
• B. $\left[N{a}^{+}\right]$ increases, but $\left[S{O}_4^{2-}\right]$ decrease.
• C. $\left[N{a}^{+}\right]$ increases, but $\left[S{O}_4^{2-}\right]$ remains the same.
• D. $\left[N{a}^{+}\right]$ and $\left[S{O}_4^{2-}\right]$ both decrease.

Answer 1

The correct option is C $\left[N{a}^{+}\right]$ increases, but $\left[S{O}_4^{2-}\right]$ remains the same.

One mole of $H_{2}S{O}_4$ gives $2$ moles of $H^{+}$ ions.

Hence if $x$ $moles$ of $H_{2}S{O}_4$ are present and $2x$ $moles$ of $NaOH$ are required for complete neutralization at equivalence point $x$ $moles$ of $H_{2}S{O}_4$ correspond to $x$ $moles$ of $S{O}_4^{2-}$.

Similarly, $2x$ $moles$ of $NaOH$ correspond to $2x$ $moles$ of $N{a}^{+}$.

During titration, $\left[N{a}^{+}\right]$ increase, but $\left[S{O}_3^{2-}\right]$ remains the same.

Finally, $\left[N{a}^{+}\right]$ becomes double of $\left[S{O}_4^{2-}\right]$.