Measuring and Comparing pH (Edexcel A Level Chemistry)

Strong and weak acids

• The relative strengths of different acids can be determined by measuring the pH of equimolar aqueous solutions of the acids, at the same temperature

• The higher the value of the pH, the weaker the acid

pH of 0.100 mol dm^-3 Aqueous Solutions of Various Acids at 298K

Strong and weak bases

pH of 0.100 mol dm^-3 Aqueous Solutions of Various Bases at 298K

• The higher the value of the pH, the stronger the base

Salts

pH of 0.100 mol dm^-3 Aqueous Solutions of Various Salts at 298K

• NaCl and KNO₃ both have pH's of 7.00 as they are made from a strong acid and a strong base
• CH₃COONa is alkaline as it is made of a weak acid (CH₃COOH) and a strong base (NaOH)
• NH₄Cl is acidic as it is made of a strong acid (HCl) and a weak base (NH₃)
• CH₃COONH₄ is neutral as it is made from a weak acid (CH₃COOH) and a weak base (NH₃) which both have similar relative strengths

Effect of dilution on the pH of aqueous solutions of acids

Strong acids

• As concentration increases by a factor of 10 the pH decreases by one unit

pH of Aqueous Solutions of Hydrochloric Acid at Different Concentrations at 298K

• Following this logic we would expect a concentration of 1.00 x 10^-8 mol dm^-3 to have a pH of 8
• This makes no sense as it would mean our acidic solution is alkaline
• However when we have solutions that are this dilute we can no longer ignore the contribution of hydrogen ions from the dissociation of water
• The hydrochloric acid with a concentration of 1.00 x 10^-8 mol dm^-3 would have a pH close to 7 as the contribution of hydrogen ions from the water is greater than that from the acid

Weak acids

• As concentration increases by a factor of 10 the pH increases by a factor of around 0.5

pH of Aqueous Solutions of Ethanoic Acid at Different Concentrations at 298K