Finding Ka Values (Edexcel International A Level (IAL) Chemistry): Revision Note

Core Practical 11: Finding Ka for a Weak Acid

• The K_a of a weak acid may be determined by finding the pH at the half equivalence point

• At the half equivalence point K_a = [H⁺]

  

Key steps in the procedure

• A pH probe or meter is calibrated before use

• 25 cm³ of 0.1 mol dm^-3 ethanoic acid is pipetted into a conical flask and a few drops of phenolphthalein indicator are added

• A burette is filled with 0.1 mol dm^-3 sodium hydroxide solution

• The contents of the conical flask are titrated against the sodium hydroxide until the indicator just turns pink

• A further pipette containing 25 cm³ of the acid is then added to the flask and the pH is measured

Specimen Results

• pH at half equivalence point = 4.75

Analysis

• After the addition of the second portion of acid, the solution is effectively a half-neutralised sample of acid

• This is the half-equivalence point at which the K_a = [H⁺]

• By measuring the pH at that point you can convert it to [H⁺] and hence find the K_a of the acid

pH = 4.75

[H⁺] = 10^-pH = 1.8 x 10^-5 mol dm^-3

∴  K_a = 1.8 x 10^-5 mol dm^-3

• You can also say that the pK_a = pH at the half-equivalence point

The pH at the half-equivalence point in a weak acid titration gives the pKa of the weak acid