A Level Chemistry 1.12 Acid and Bases Part II

Explain why chloroethanoic acid is a stronger acid than ethanoic acid (2)

Electronegative chlorine withdraws electrons (1)

stabilises/reduces charge on COO^- (1)

Explain why data book do not usually contain values of Ka for strong acids (2)

Strong acids (almost) completely dissociate (1)

Ka value for strong acids is very large (1)

Two solutions, one with pH of 4.00 and the other with a pH of 9.00, were left open to the air.

The pH of the pH 9.00 solution changed more than that of the other solution.

Suggest what substance might be present in the air to cause pH to change.

Explain how and why the pH of the pH 9.00 solution changes (3)

Carbon dioxide (1)

CO₂ is used to form carbonate ions/hydrogen carbonate (1)

pH increases (1)

The pH curve shown below was obtained when a 0.150 mol dm^–3 solution of sodium hydroxide was added to 25.0 cm³ of an aqueous solution of a weak monoprotic acid, HA.

Use the information given to calculate the concentration of the acid (2)

Concentration of acid : m₁v₁ = m₂v₂ (1)

25×m₁ = 18.2×0.150 (1)

m₁ = 18.2×0.150/25= 0.109 (10

Titration curves labelled A, B, C and D for combinations of different acids and bases are shown below. All solutions have a concentration of 0.1 mol dm^–3.

Select from A, B, C and D the curve produced by the addition of

ammonia to 25 cm³ of hydrochloric acid  _____________________________

ethanoic acid to 25 cm³ of sodium ________________________

sodium hydroxide to 25 cm³ of hydrochloric acid  ______________________ (3)

B (1)

C (1)

A (1)

Describe briefly how you would ensure that a reading from a pH meter is accurate. (2)

Calibrate meter with solutions of known pH/buffer (1)

Adjust meter/plot calibration curve (1)

Calculate the concentration, in mol dm^–3, of an aqueous solution of sulfuric acid that has a pH of 0.25 (2)

[H⁺] = 0.56 (1)

[H₂SO₄] = ½ × 0.56 = 0.28 (1)