Answer:
Approximately [tex]2\times 10^{-12}\;\rm mol \cdot L^{-1}[/tex].
Explanation:
The hydronium ion concentration [tex]\left[\mathrm{H_3O^{+}}\right][/tex] of an aqueous solution can be found from its [tex]\rm pH[/tex] with the equation:
[tex]\displaystyle \left[\mathrm{H_3O^{+}}\right] = 10^{-\mathrm{pH}}[/tex].
For this solution, [tex]\rm pH= 11.7[/tex]. Hence,
[tex]\begin{aligned}& \left[\mathrm{H_3O^{+}}\right] \\ &= 10^{-\mathrm{pH}} \\ &= 10^{-11.7} \approx 2 \times 10^{-12}\end{aligned}[/tex].
Note that for this equation, the number of significant figures in [tex]\left[\mathrm{H_3O^{+}}\right][/tex] should be the same as the number of decimal places in [tex]\rm pH[/tex]. For example, the [tex]\rm pH[/tex] of this question comes with only one decimal place. As a result, there would be only one significant figure in the [tex]\left[\mathrm{H_3O^{+}}\right][/tex] obtained from the equation.
