Question

Calculate the concentration of OH in a solution that contains 3.9 10-4 M H30 at 25°C. Identify the solution as acidic, basic or neutral. A) 2.6 x 10-11 M, acidic OB) 2.6x 10-11 M, basic OC) 3.9x 10-4 M, neutral D) 2.7 10-2 M, basic

Answer 1

Answer:

Option A) 2.6 × 10⁻¹¹ M, acidic.

Explanation:

The ion-product constant of water $K_{\rm w}$ gives a relationship between $\rm [H_3O^{+}]$ and $\rm [OH^{-}]$:

$K_{\rm w} = \rm [H_3O^{+}]\cdot [OH^{-}]$.

The exact value of $K_{\rm w}$ depends on the temperature. Under $\rm 25^{\circ}C$, $K_{\rm w} \approx 1^{-14}$.

The question states that $\rm [H_3O^{+}] = 3.9\times 10^{-4}\; M$. As a result,

$\begin{aligned} {\rm [OH^{-}]} &= \frac{K_{\rm w}}{[{\rm H_3O^{+}}]}\\ &\approx\frac{1^{-14}}{3.9\times 10^{-4}} && \rm {\leftarrow Only~under~25^{\circ}C.}\atop{}\\&\approx \rm 2.6\times 10^{-11}\; M\end{aligned}$.

• A solution is acidic if $\rm [H_3O^{+}] > [OH^{-}]$.
• A solution is neutral if $\rm [H_3O^{+}] = [OH^{-}]$.
• A solution is acidic if $\rm [H_3O^{+}] < [OH^{-}]$.

$\rm [H_3O^{+}] > [OH^{-}]$ for this solution. As a result, this solution is acidic.