Question

Given pH = 8.55 Find: [H3O+] and [OH-] Is this acidic, basic or neutral?​

Answer 1

Answer:

• $\rm [H_3O^{+}] = 10^{-8.55}\;M\approx 2.82\times 10^{-9} \;M$, and
• $\rm [OH^{-}] = 10^{-5.45}\; M\approx 3.55\times 10^{-6}\;M$

This solution is likely to be basic.

Assumption: this solution is under room temperature, where $K_w = 10^{-14}$.

Explanation:

The concentration of hydronium ions $\mathrm{H_3O^{+}}$ in the solution can be found from the $\mathrm{pH}$ value. This relationship does not depends on temperature.

$\mathrm{[H_3O^{+}]} = 10^{-\mathrm{pH}} = 10^{-8.55}$.

The question states that for this solution, $\rm [H_3O^{+}] = 8.55\;M$. Apply the relationship between $\mathrm{[H_3O^{+}]}$, $\mathrm{[OH^{-}]}$, and $K_w$. Note that the value of $K_w$ is dependent on the temperature of the solution.

$\displaystyle \rm [OH^{-}] = \frac{\mathnormal{K_w}}{[H_3O^{+}]} = \frac{10^{-14}}{10^{-8.55}} = 10^{-5.45}\; M$.

In other words,

• $\rm [H_3O^{+}] = 10^{-8.55}\;M\approx 2.82\times 10^{-9} \;M$, and
• $\rm [OH^{-}] = 10^{-5.45}\; M\approx 3.55\times 10^{-6}\;M$ assuming that $K_w = 10^{-14}$.

$\rm [H_3O^{+}] < [OH^{-}]$.

In other words, this solution is basic.