# How do you calculate pH of acid and base solution?

Mar 19, 2014

Sorry, but this might get lengthy, so bear with me please :)

$\left[p H\right]$(http://socratic.org/chemistry/acids-and-bases/the-ph-concept), or potential of Hydrogen, is measured on a scale from 0 to 14. 0 being the most ACIDIC, while 14 being the most BASIC.

To find $p H$ from the concentration of ${H}_{3} {O}^{+}$ (or just simply ${H}^{+}$) you need to do the following:

$p H$= -log[${H}_{3} {O}^{+}$]

The [${H}_{3} {O}^{+}$] is just the concentration (in molarity) found through calculations (I'll cover that soon)

If you have the concentration of $O {H}^{-}$, however, simply find the $p O H$ by:

$p O H$=-log[$O {H}^{-}$]

After you get this number, you do the following:

$p H$= 14-$p O H$

OK, so lets start with the basics of determining your ${H}_{3} {O}^{+}$ or $O {H}^{-}$ concentrations. Molarity is the standard unit for concentration in chemistry, and is simply moles of substance over liters of solution.
$M = \frac{m o l}{v o l u m e \left(L\right)}$
So whenever I say concentration, I mean Molarity.

You find the concentration of ${H}_{3} {O}^{+}$ by first writing out your acid dissociation equation:
$H A$+ ${H}_{2} O$= ${H}_{3} {O}^{+}$ + ${A}^{-}$
...where $H A$ is simply the acid you're dissolving in water.

If you have a STRONG Acid, then it dissociates (dissolves) completely in water. The concentration of ${H}_{3} {O}^{+}$ is the same as the concentration of the initial acid.

Now, you were probably given the $K a$ of the acid, telling you that it is a WEAK acid. That means that it does NOT dissociate (dissolve) completely in water. The Ka at this point is just a number to plug into your equation.

To find the concentration of ${H}_{3} {O}^{+}$ from the $K a$ and your equation, simply plug the numbers that you have into this: You can do the exact same thing if its a BASIC solution, just replace [${H}_{3} {O}^{+}$] with [$O {H}^{-}$] and dont forget to change the $p O H$ to $p H$