Acids and bases (A-level only) common mistakes

Common mistakes

• Confusing acids and bases

  Students often confuse Brønsted-Lowry acids with bases and vice versa.

  Fix itRemember that acids donate protons while bases accept protons.

• Confusing conjugate acids and bases

  Students often confuse which species is the conjugate acid and which is the conjugate base.

  Fix itRemember that the conjugate acid is formed when a base gains a proton, while the conjugate base is formed when an acid loses a proton.

• Confusing acids and bases

  Students often confuse the roles of acids and bases in reactions.

  Fix itRemember that acids donate protons and bases accept protons.

• Confusing acid strength with concentration

  Students often think that a concentrated acid is necessarily a strong acid.

  Fix itRemember that strength refers to the degree of ionization, while concentration refers to the amount of acid in solution.

• Confusing pH with hydrogen ion concentration

  Students often confuse pH values with hydrogen ion concentrations, thinking they are directly proportional.

  Fix itRemember that pH is the negative logarithm of hydrogen ion concentration; as [H⁺] increases, pH decreases.

• Confusing pH with pOH

  Students often confuse pH and pOH, leading to incorrect calculations of ion concentrations.

  Fix itRemember that pH + pOH = 14 at 25°C, and use this relationship to find the correct values.

• Incorrectly calculating [OH⁻] from pH

  Students often forget to convert pH to [H⁺] correctly before using Kw.

  Fix itAlways use [H⁺] = 10^-pH to find the hydrogen ion concentration before calculating [OH⁻].

• Confusing pH and pOH

  Students often confuse pH and pOH calculations.

  Fix itRemember that pH + pOH = 14 at 25°C. Always calculate one before the other if needed.

• Incorrect Ka Expression

  Students often confuse the placement of concentrations in the Ka expression.

  Fix itRemember that Ka = [H⁺][A⁻] / [HA], where the products are in the numerator and the reactants in the denominator.

• Ignoring the approximation in weak acid calculations

  Assuming that the initial concentration of the weak acid remains unchanged when calculating [H⁺].

  Fix itAlways check if the change in concentration due to dissociation is negligible compared to the initial concentration.