Calculations involving pH

The pH of a solution depends on the concentration of H⁺(aq) ions present.

A shorthand method of denoting concentration of hydrogen ions is [H⁺(aq)]. The square brackets surrounding the ion symbol mean 'concentration in moles per litre'.

When the concentration of hydrogen ions is expressed using powers of 10, the pH is equal to the negative of the power number.

Example:

[H⁺(aq)] = 0.01 mol l^-1 = 10^-2 mol l^-1

So in the example above, the pH = 2. This is because the concentration of H⁺ ions are expressed as a power of 10, where the power is -2.

The exact pH of a solution can be calculated using:

    pH = -log[H⁺(aq)]

For most solutions the pH lies in the range 0 to 14.

Example:

In a solution of pH 2, what is the concentration of H⁺(aq)?

pH of solution = 2

[H⁺(aq)] = 10^-2 mol l^-1

Example:

A salt solution has a hydrogen ion concentration of 10^-9 mol l^-1. What is its pH?

[H⁺(aq)] = 10^-9 mol l^-1

pH of solution = 9