DP Chemistry Questionbank

Which mixture is a buffer solution?

A.  25 cm³ of 0.10 mol dm^-3 NH₃ (aq) and 50 cm³ of 0.10 mol dm^-3 HCl (aq)

B.  50 cm³ of 0.10 mol dm^-3 NH₃ (aq) and 25 cm³ of 0.10 mol dm^-3 HCl (aq)

C.  25 cm³ of 0.10 mol dm^-3 NaOH (aq) and 25 cm³ of 0.10 mol dm^-3 HCl (aq)

D.  50 cm³ of 0.10 mol dm^-3 NaOH (aq) and 25 cm³ of 0.10 mol dm^-3 HCl (aq)

Which salt solution has the highest pH?

A. NH₄Cl

B. Ca(NO₃)₂

C. Na₂CO₃

D. K₂SO₄

(i) Sketch a graph of pH against volume of a strong base added to a weak acid showing how you would determine pK_a for the weak acid.

(ii) Explain, using an equation, why the pH increases very little in the buffer region when a small amount of alkali is added.

A buffer is produced by mixing 20.0 cm³ of 0.10 mol dm⁻³ ethanoic acid, CH₃COOH(aq), with 0.10 mol dm⁻³ sodium hydroxide, NaOH(aq).

What is the volume of NaOH required and the pH of the buffer?

Suggest a suitable indicator for the titration of hydrazine solution with dilute sulfuric acid using section 22 of the data booklet.

The following curve was obtained using a pH probe.

State, giving a reason, the strength of the acid.

Deduce the pK_a for this acid.

Suggest, giving a reason, which point on the curve is considered a buffer region.

Identify the major species, other than water and potassium ions, at these points.

Which indicator is appropriate for the acid-base titration shown below?

A. Thymol blue (pK_a = 1.5)
B. Methyl orange (pK_a = 3.7)
C. Bromophenol blue (pK_a = 4.2)
D. Phenolphthalein (pK_a = 9.6)

Sketch the general shape of the variation of pH when 50 cm³ of 0.001 mol dm^–3 NaOH (aq) is gradually added to 25 cm³ of 0.001 mol dm^–3 CH₃CH₂COOH (aq).

Identify if aqueous solutions of the following salts are acidic, basic, or neutral.

Identify an indicator that could be used for the titration in 5(d)(i), using section 22 of the data booklet.

Explain, using appropriate equations, how a suitably concentrated solution formed by the partial neutralization of 2,2-dimethylpropanoic acid with sodium hydroxide acts as a buffer solution.

The graph represents the titration of 25.00 cm³ of 0.100 mol dm⁻³ aqueous ethanoic acid with 0.100 mol dm⁻³ aqueous sodium hydroxide.

Deduce the major species, other than water and sodium ions, present at points A and B during the titration.

Predict whether the pH of an aqueous solution of ammonium chloride will be greater than, equal to or less than 7 at 298 K.

Sketch the neutralisation curve obtained and label the equivalence point.

Sketch the pH curve for the titration of 25.0 cm³ of ethylamine aqueous solution with 50.0 cm³ of butanoic acid aqueous solution of equal concentration. No calculations are required.

An indicator, HIn, has a pK_a of 5.1.

HIn (aq) $\rightleftharpoons$ H⁺ (aq) + In⁻ (aq)

colour A                      colour B

Which statement is correct?

A.   At pH = 7, colour B would be observed
B.   At pH = 3, colour B would be observed
C.   At pH = 7, [HIn] = [In⁻]
D.   At pH = 3, [HIn] < [In⁻]

An aqueous solution containing high concentrations of both NH₃ and NH₄⁺ acts as an acid-base buffer solution as a result of the equilibrium:

NH₃ (aq) + H⁺ (aq) $\rightleftharpoons$ NH₄⁺ (aq)

Referring to this equilibrium, outline why adding a small volume of strong acid would leave the pH of the buffer solution almost unchanged.

Suggest a suitable indicator for the titration, using section 22 of the data booklet.

Sketch a graph of pH against volume of hydrochloric acid added to ammonia solution, showing how you would determine the pK_a of the ammonium ion.

Explain, using two equations, how an equimolar solution of ammonia and ammonium ions acts as a buffer solution when small amounts of acid or base are added.

Aqueous sodium hydrogencarbonate has a pH of approximately 7 at 298 K.

Sketch a graph of pH against volume when 25.0cm³ of 0.100 mol dm⁻³ NaOH (aq) is gradually added to 10.0cm³ of 0.0500 mol dm⁻³ NaHCO₃ (aq).

Where is the buffer region for the titration of a weak acid with a strong base?

Describe, using a suitable equation, how the buffer solution formed during the titration resists pH changes when a small amount of acid is added.