How do you calculate something on a pH scale?

1 Answer
Alex D. · Ernest Z. · Media Owl · mason m
Mar 18, 2014

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

Explanation:

pH, or potential of hydrogen, is measured on a scale from 0 to 14, with 0 being the most ACIDIC and 14 the most BASIC.

To find pH from the concentration of H3O+ (or just simply H+) you need to use the formula:

pH=−log[H3O+]

The [H3O+] is just the concentration (in molarity) found through calculations (I'll cover that soon).

If you have the concentration of OH−, however, simply find the pOH from the expression:

pOH=−log[OH−]

After you get this value, you use the formula:

pH= 14-pOH

OK, so let's start with the basics of determining your H3O+ or OH− concentrations.

Molarity is the standard unit for concentration in chemistry, and is simply moles of substance over liters of solution.

M=molesliters

So whenever I say concentration, I mean molarity.

You find the concentration of H3O+ by first writing out your acid dissociation equation:

HA+H2O⇌H3O++A−

… where HA is simply the acid you're dissolving in water.

If you have a STRONG acid, then it dissociates completely in water.

The concentration of H3O+ is the same as the concentration of the initial acid.

Now, you were probably given the Ka of the acid, telling you that it is a WEAK acid.

That means that it does NOT dissociate completely in water.

The Ka at this point is just a number to plug into your equation.

To find the concentration of H3O+ from the Ka and your equation, simply plug the numbers that you have into this expression:

Ka=[A−][H3O+][HA]

You can do the exact same thing if it's a BASIC solution.

Just replace [H3O+] with [OH−] and don't forget to change the pH to pOH.

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