Acid-base Equilibria (Edexcel International A Level (IAL) Chemistry): Exam Questions

Sodium hydrogensulfate is a widely used acid, with applications that include removing limescale and as a food additive. Sodium hydrogensulfate is a weak acid because of the presence of the hydrogensulfate ion, HS${\mathrm{O}}_4^{-}$ .

i) Write the equation for the dissociation of the hydrogensulfate ion in aqueous solution. State symbols are not required.

(1)

ii) A solution of sodium hydrogensulfate has pH = 1.13 Calculate the concentration of this solution, in g dm⁻³.
[pK_a of HS${\mathrm{O}}_4^{-}$= 1.92]

(5)

iii) State the assumptions you have used in (a)(ii).

(2)

A solution containing sodium hydrogensulfate and sodium sulfate is a buffer that is used to preserve urine for steroid analysis.

i) State what is meant by the term buffer.

(2)

ii) Calculate the pH of the buffer prepared by dissolving 0.750 mol of sodium hydrogensulfate and 0.500 mol of sodium sulfate in distilled water to make 1.00 dm³ of solution.

(3)

iii) Separate samples of 0.00500 mol of hydrochloric acid are added to 1.00 dm³ of distilled water and to the buffer in (b)(ii).

Calculate the pH changes that result in each case. Assume that the volumes remain constant at 1.00 dm³.

(4)

The titration curve obtained from the addition of sodium hydroxide solution to a weak acid is shown. The equivalence point (E) of this titration occurred at pH = 8

Explain the observations that would be made if methyl orange (pK_In = 3.7) were used as the indicator for this titration.

A 0.100 mol dm⁻³ solution of ammonium chloride, NH₄Cl, has a pH of 5.13 at 298 K.

The ammonium ion acts as a weak acid in aqueous solution:

NH₄⁺ (aq) ⇌ NH₃ (aq) + H⁺ (aq)

Calculate the value of K_a for the ammonium ion at 298 K.

Give your answer to 2 significant figures.

Calculate the mass of ammonium chloride, NH₄Cl, that must be added to 100 cm³ of 0.100 mol dm⁻³ aqueous ammonia to prepare a buffer of pH 9.50.

Give your answer to an appropriate number of significant figures.

[Assume the volume change on adding NH₄Cl is negligible. Use your value of K_a(NH₄⁺) from part (a), or K_a(NH₄⁺) = 5.5 × 10^-10 mol dm^-3.]

The buffer prepared in part (b) is diluted by a factor of 10 by adding distilled water.

State the effect this dilution has on the pH of the solution. Use the K_a expression to explain your answer.

Using equations, explain how the diluted buffer still resists a change in pH when small amounts of acid or alkali are added.