Question

Calculate the hydronium ion concentration in an aqueous solution with a pH of 11.7 at 25°C.

Answer 1

Answer:

Approximately $2\times 10^{-12}\;\rm mol \cdot L^{-1}$.

Explanation:

The hydronium ion concentration $\left[\mathrm{H_3O^{+}}\right]$ of an aqueous solution can be found from its $\rm pH$ with the equation:

$\displaystyle \left[\mathrm{H_3O^{+}}\right] = 10^{-\mathrm{pH}}$.

For this solution, $\rm pH= 11.7$. Hence,

$\begin{aligned}& \left[\mathrm{H_3O^{+}}\right] \\ &= 10^{-\mathrm{pH}} \\ &= 10^{-11.7} \approx 2 \times 10^{-12}\end{aligned}$.

Note that for this equation, the number of significant figures in $\left[\mathrm{H_3O^{+}}\right]$ should be the same as the number of decimal places in $\rm pH$. For example, the $\rm pH$ of this question comes with only one decimal place. As a result, there would be only one significant figure in the $\left[\mathrm{H_3O^{+}}\right]$ obtained from the equation.