Question

At 25°C, the [$OH^{-}$] is $10^{-9}$. Find out the value of pH of the solution.

Answer 1

Given,  

At 25°C, the concentration of ${ OH }^{ - } is { 10 }^{ -9 }$ in the solution

We know that,

At room temperature,

${ K }_{ w }\quad =\quad \left[ { H }^{ + } \right] \left[ { OH }^{ - } \right] \quad =\quad { 10 }^{ (-14) }$

It is given that, $\left[ { OH }^{ - } \right] \quad =\quad { 10 }^{ -9 }, hence \left[ { H }^{ + } \right] \quad =\quad { 10 }^{ -5 }$

So, $pH\quad =\quad -log\left[ { H }^{ + } \right] \quad =\quad 5$

But pH + pOH = 14

So, pH of the solution is 5.