Question

Which of the following $pH$ measurements is the weakest acid; a $pH$ of $4$, a $pH$ of $6$, or a $pH$ of $8$?

Answer 1

We know that $pH$ is a measure of the concentration of the hydrogen ions i.e. it is a measure of the acidity or alkalinity of a given solution. The$pH$that corresponds to the least hydronium ion concentration will be the $pH$of the weakest acid.

Complete answer:
We know that water undergoes autoprotolysis. During autoprotolysis reaction, a proton is transferred from one molecule of the solvent to another molecule of the solvent which results in a positive ion and a negative ion. The autoprotolysis of water is as follows:
$2{H_2}O \rightleftharpoons {H_3}{O^ + } + H{O^ - }$
Under standard conditions of temperature and pressure, the equilibrium expression is:
${K_w}$ is $[{H_3}{O^ + }][H{O^ - }] = {10^{ - 14}}$.
Take ${\log _{10}}$on both sides of the equation:
${\log _{10}}\left( {{K_w}} \right)$ is ${\log _{10}}[{H_3}{O^ + }] + {\log _{10}}[H{O^ - }] = {\log _{10}}\left( {{{10}^{ - 14}}} \right)$
By rearranging the equation, we get:
$\Rightarrow - {\log _{10}}[{H_3}{O^ + }] - {\log _{10}}[H{O^ - }] = 14$
Here, ${\log _{10}}[{H_3}{O^ + }]$ is the $pH$ and ${\log _{10}}[H{O^ - }]$ is the $pOH$. Here,$pOH$is the measure of the concentration of the hydroxide ions present in the solution. It is related to the concentration of hydrogen ions present in a solution.
The lower the numerical $pH$, the greater the concentration of the hydronium ions. So, at $pH = 8$, the concentration of hydronium ions will be less than the concentration of $[H{O^ - }]$.
Therefore, at $pH = 8$, the solution is the weakest acid.

Note:
The $pH$ scale is generally logarithmic and it inversely indicates the concentration of hydrogen ions present in a solution. This is because the formula of $pH$approximates the negative of the base $10$logarithm of molar concentration of the hydrogen ions present in the solution.