pH and pOH pH The pH scale pH is a measure of the acidity of a solution. It is calculated by determining the concentration of hydrogen ions in the solution. In water, hydrogen ions (H+) exist as hydronium ions (H3O+).

pH is defined as the negative logarithm of the hydronium ion concentration:

and

Note: = concentration of hydronium ions (mol.L-1)

Since the pH scale is a logarithmic scale increasing the concentration of H3O+ by a factor of 10 results in a decrease of one pH unit.

pH
1
2
3
4
5
6
7
8
9
10
11
12
13
14
[H3O+]
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-12
10-13
10-14

0.1
0.01
0.001
0.0001
0.00001 etc Calculating the pH of Strong acids
{Calculate the pH of strong acids given hydrogen ion concentration.}

When a strong acid, such as hydrochloric acid is added to water, it fully ionises, leaving no HCl molecules in the solution.

Thus the concentration of hydronium ions in the solution will be equal to the initial hydrochloric acid concentration.

This concentration can be used to calculate the pH of the solution.

Model: Determine the pH of a 0.1 mol.L-1 HCl solution.
Solution: As HCl is a strong acid it fully ionises. Therefore:

Note: Calculation of the pH of weak acid solution is not within the scope of this course. This calculation is more difficult as weak acids do not fully ionise.

Students do:
Calculate the pH of the following solutions. Show full working out.
0.05 molL-1 HCl (Ans: 1.3)
0.5 molL-1 HNO3 (Ans: 0.3)
[H3O+] = 1 X 10-3 molL-1 (Ans: 3)
[H3O+] = 0.486 molL-1 (Ans: 0.313)
[H3O+] = 1 X 10-7 molL-1 (Ans: 7)

Model: Calculate the concentration of a solution of nitric acid which has a pH of 1.5
Solution:

HNO3 is a strong acid which fully ionises. Therefore:

Alternative: (for those not comfortable with rearranging log equations)

HNO3 is a strong acid which fully ionises. Therefore:

Students do:
Calculate the concentration of the following solutions: pH = 6 (Ans: 1 x 10-6 mol.L-1) pH = 1.3 (Ans: 0.050 mol.L-1) pH = 4.0