MCQ
The value of the ion product constant for water, $(K_w )$ at $60\,^oC$ is $9.6 \times 10^{-14} \,M^2$. What is the $[H_3O^+]$ of a neutral aqueous solution at $60\,^oC$ and an  aqueous solution with a $pH = 7.0$ at $60\,^oC$ are respectively ?
  • A
    $3.1 \times 10^{-8}$, acidic
  • B
    $3.1 \times 10^{-7}$, neutral
  • C
    $3.1 \times 10^{-8}$, basic
  • $3.1 \times 10^{-7}$, basic

Answer

Correct option: D.
$3.1 \times 10^{-7}$, basic
d
Solution:- (D) $3.1 \times 10^{-7}$, basic

Given:$\mathrm{K}_{\mathrm{w}}=9.6 \times 10^{-14} \mathrm{M}^{2}$

As we know that,

for a neutral solution,

$\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]=\left[\mathrm{OH}^{-}\right]$

Now,

$\mathrm{K}_{\mathrm{w}}=\left[\mathrm{H}_{3} \mathrm{O}^{+}\right] \times\left[\mathrm{OH}^{-}\right]$

$\Rightarrow \mathrm{K}_{\mathrm{w}}=\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]^{2}\left(\because\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]=\left[\mathrm{OH}^{-}\right]\right)$

$\Rightarrow\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]^{2}=9.6 \times 10^{-14}$

$\Rightarrow\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]=\sqrt{9.6 \times 10^{-14}} \approx 3.1 \times 10^{-7}$

Hence at $60^{\circ} \mathrm{C}$, for a neutral aqueous solution, the $\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]$ is $3.1 \times 10^{-7}$.

Now, pH of the neutral solution-

$\mathrm{pH}=-\log \left[\mathrm{H}_{3} \mathrm{O}^{+}\right]$

$\mathrm{pH}=-\log \left(3.1 \times 10^{7}\right)$

$\Rightarrow \mathrm{pH}=(7-\log (3.1))=6.51$

since pH of a neutral solution is less than 7 , the aqueous solution with a $\mathrm{pH}=7$ is basic in nature.

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