# The pH of a solution is 2.57 at 25 degrees C. What is the molar concentration of hydronium ions? What is the molar concentration of hydroxide ions?

## The pH of a solution is 2.57 at 25 degrees C. What is the molar concentration of hydronium ions? What is the molar concentration of hydroxide ions?

Sep 7, 2016

Given the definition of $p H$, I can write immediately that if $p H = 2.57$, then $\left[{H}_{3} {O}^{+}\right] = {10}^{- 2.57}$ $m o l \cdot {L}^{-} 1$

#### Explanation:

And also given the definitions, I know that $\left[{H}_{3} {O}^{+}\right] \left[H {O}^{-}\right] = {10}^{-} 14$ under standard condtions; taking ${\log}_{10}$ of both sides, this gives our defining relationship:

$p H + p O H = 14$

And thus $p O H$ $=$ $\left\{14 - 2.57\right\}$, and $H {O}^{-}$ $=$ ${10}^{- 11.43} \cdot m o l \cdot {L}^{-} 1$.

Of course you will need a calculator to assess these values. I have never got one handy.

In a neutral solution at $298 \cdot K$, the equilibrium operates, $\left[{H}_{3} {O}^{+}\right] \left[H {O}^{-}\right] = {10}^{-} 14$, and $p H$ $=$ $p O H$ $=$ $7$. At temperatures higher than $298 K$, how would you expect $p H$ to evolve in a neutral solution. Would it increase, decrease, stay the same?