Definition & Determination of pH (Oxford AQA International A Level Chemistry)

pH Scale Calculations

• The pH scale is a numerical scale that shows how acidic or alkaline a solution is

• The values on the pH scale go from 0-14 (extremely acidic substances have values of below 1)

  • All acids have pH values below 7

  • All alkalis have pH values above 7

• The pH scale is a measure of the concentration of hydrogen ions in a solution

  • The lower the pH, the higher the concentration of hydrogen ions and the more acidic the solution is

  • The higher the pH, the lower the concentration of hydrogen ions and the more alkaline the solution is

The pH scale

Calculating pH

• The pH is defined as:

pH = -log[H⁺]

where [H⁺] is the concentration of hydrogen ions in mol dm^–3

• Similarly, the concentration of H⁺ of a solution can be calculated if the pH is known by rearranging the above equation to:

[H⁺] = 10^-pH

• The pH scale is a logarithmic scale with base 10

• This means that each value is 10 times the value below it

  • For example, pH 5 is 10 times more acidic than pH 6

• pH values are usually given to 2 decimal places

• The relationship between concentration is easily seen on the following table:

Calculating the pH of strong acids

• Strong acids are completely ionised in solution

HA (aq) → H⁺ (aq) + A^- (aq)

• Therefore, the concentration of hydrogen ions, H⁺, is equal to the concentration of acid, HA

• The number of hydrogen ions formed from the ionisation of water is very small relative to the [H⁺] due to ionisation of the strong acid and can therefore be neglected

• The total [H⁺] is therefore the same as the [HA]