Chemistry Notes: pH Calculations

Chemistry 2014-2015

We have already defined pH as a measurement of the concentration of hydrogen ions in a solution, and pOH as the concentration of hydroxide ions in a solution. The pH and pOH of a substance add up to 14.

pH and pOH Calculations

We can actually calculate pH and pOH from their respective ion concentrations using the equations below.pH = -log[H+] pOH = -log[OH-]

We can also calculate the concentration of hydrogen and hydroxide ions from their respective pH or pOH.

[H+] = 10-pH [OH-] = 10-pOH

Lastly, we can calculate pH from pOH (or vice-versa) and hydrogen ion concentration from hydroxide ion concentration (or vice-versa).

pH + pOH = 14 [H+]*[OH-] = 10-14

For example, a solution with a pH of 3.0 has a pOH of 11.0. Its hydrogen ion concentration is 1.0 x 10-3 M and its hydroxide ion concentration is 1.0 x 10-11 M.

pOH = 14 – pH = 14 – 3.0 = 11.0[H+] = 10-pH = 10-3.0 = 1.0 x 10-3

[OH-] = 10-pOH = 10-11.0 = 1.0 x 10-11

What is the pH of this solution?

What is the pOH of this solution?

What is the hydroxide ion concentration of this

solution?

Is this solution an acid or a base?

A solution has a hydrogen ion concentration of 2.3 x 10-9 M.

 What is the pH of this solution?

What is the hydroxide ion concentration of this solution? What is the hydrogen ion concentration of this solution? Is this solution an acid or a base?

A solution has a pOH of 11.5