pH (Edexcel A Level Chemistry): Revision Note

Defining pH

• The acidity of an aqueous solution depends on the number of H⁺ ions in solution

• 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

pH & [H+] Table

pH & Hydrogen Ion Concentration

Answer

Answer 1:

The pH of the solution is:

   pH  = -log[H⁺]

         = -log 1.6 x 10^-4

         = 3.80

Answer 2:

The hydrogen concentration can be calculated by rearranging the equation for pH

   pH   = -log[H⁺]

   [H⁺] = 10^-pH

          = 10^-3.10

          = 7.94 x 10^-4 mol dm^-3

Answer

The correct option is C.

• The total volume after dilution is 1000.0 cm³ so the concentration of H⁺ has been reduced by a factor of 100 or 10^-2, which means an increase of 2 pH units

  • The final solution is therefore pH 3