# Soap is made from a strong base and oil. Do you think the pH of soap is 4 or 9?

Oct 31, 2016

Should not you tell us? $p H = - {\log}_{10} \left[{H}_{3} {O}^{+}\right]$ by definition.

#### Explanation:

In water, autoprotolysis occurs, and $\left[{H}_{3} {O}^{+}\right] \left[H {O}^{-}\right] = {10}^{-} 14$ under standard conditions.

If we take $- {\log}_{10}$ of both sides of the expression:

$- {\log}_{10} \left[{H}_{3} {O}^{+}\right] - {\log}_{10} \left[H {O}^{-}\right] = - {\log}_{10} {10}^{-} 14$

But $- {\log}_{10} {10}^{-} 14 = 14$ by definition, because ${\log}_{a} B$ is the power to which we raise the base $a$ to get $B$.

Also by definition, $p H = - {\log}_{10} \left[{H}_{3} {O}^{+}\right]$ and $p O H = - {\log}_{10} \left[H {O}^{-}\right]$.

Thus $p H + p O H = 14$

At neutrality, $\left[H {O}^{-}\right] = \left[{H}_{3} {O}^{+}\right]$, and thus $p H = p O H = 7$, that is the concentrations of hydroxide ion and hydronium ions are EQUAL. If $p H < 7$, the solution is acidic, and $p H > 7$, the solution is basic. Your $p H = 9$, which suggest what with respect to $\left[H {O}^{-}\right]$?

It is worthwhile getting these definitions right. They are fairly simple and logical, and problems based on $p H$ can become routine. If you have a problem, or if I have glossed over a step you do not understand, ask for clarification and someone here will help you.