Acids & Bases — practice questions

Give the conjugate base of HF and of NH₄⁺, and the conjugate acid of NH₃ and of CH₃COO⁻.

Explain, with equations, why a solution of ammonium chloride, NH₄Cl, is acidic while a solution of sodium chloride is neutral.

Hydrogen sulfide, H₂S, has Ka = 8.9 × 10⁻⁸ for its first dissociation. Sodium hydrogen sulfide, NaHS, dissolves to give the HS⁻ ion, which is amphiprotic. Write equations showing HS⁻ acting as both an acid and a base, identify the conjugate species in each, and explain what additional information would be needed to predict whether a solution of NaHS is acidic or basic.

Explain the difference between a strong acid and a concentrated acid, and write equations showing the behaviour of HCl and CH₃COOH in water.

Equal volumes of 0.10 mol L⁻¹ hydrochloric acid and 0.10 mol L⁻¹ ethanoic acid are each reacted with excess magnesium. Explain how the initial rates of reaction and the total volumes of hydrogen produced compare.

A student measures the pH of 0.10 mol L⁻¹ solutions of HCl, CH₃COOH and NH₄Cl as 1.00, 2.88 and 5.13 respectively. They conclude that 'HCl is the most concentrated acid and NH₄Cl is the most dilute'. Evaluate this conclusion, calculate the percentage dissociation of each acid, and explain what the results reveal about the relative strengths of the three.

Calculate the pH of 0.0150 mol L⁻¹ nitric acid, HNO₃, and state whether the solution is acidic or basic.

Calculate the pH of 0.0220 mol L⁻¹ potassium hydroxide, KOH, and explain why the calculation requires an extra step compared with finding the pH of a strong acid.

A student dilutes 10.0 mL of 0.100 mol L⁻¹ HCl to 1.00 L and correctly finds the new pH to be 3.00. They then propose that diluting this solution by a further factor of 10 000 would give a pH of 7.00, and diluting again would make it basic. Evaluate this reasoning.

Calculate the pH of 0.200 mol L⁻¹ hydrofluoric acid, HF, given Ka = 6.8 × 10⁻⁴.

Calculate the pH of 0.250 mol L⁻¹ methylamine, CH₃NH₂, given Ka(CH₃NH₃⁺) = 2.3 × 10⁻¹¹. Explain why Kb had to be derived.

A student calculates the pH of 1.0 × 10⁻⁴ mol L⁻¹ ethanoic acid (Ka = 1.74 × 10⁻⁵) using the standard method and obtains pH 4.38. Evaluate whether the standard approximation is valid at this concentration, and explain what this reveals about the conditions under which the method may be used.

List all the species present in a 0.10 mol L⁻¹ solution of ethanoic acid, in order of decreasing concentration. Do not include water.

List the species present in a 0.10 mol L⁻¹ solution of ammonium chloride in order of decreasing concentration, and explain the position of each in your order.

Two solutions have the same pH of 3.0: one is 1.0 × 10⁻³ mol L⁻¹ HCl and the other is a more concentrated solution of a weak acid HA. Compare and contrast the two solutions in terms of the species present, their relative concentrations, their electrical conductivity, and the volume of 0.10 mol L⁻¹ NaOH each would require for complete neutralisation.