CHAPTER 6B

Which of the following species can act as an Bronsted acid as well as Bronsted base?

• HCl
• H₂SO₄
• HSO₃^-
• SO₃^2-

Which of the following shaded species is a Lewis acid?

• CO₂ + OH^_-   → HCO₃^-
• AlCl₃ + Cl- → AlCl₄^-
• B (OH)₃ + OH^- → B(OH)₄^-
• 2 NH₃ + Ag⁺ → Ag(NH₃)₂⁺

The strength of a weak acid is usually compared by using its

• acid dissociation constant
• degree of dissociation
• pH value
• concentration

The pH range of indicator thymol blue is 8.0 - 9.6.  It can be deduced that thymol blue is a suitable indicator for the titration of 

• H₂SO₄ (aq) with NaOH (aq)
• H₂SO₄ (aq) with NH₃ (aq)
• CH₃COOH (aq) with NH₃ (aq)
• FeSO₄ (aq) with KMnO₄ (aq)

Consider  a buffer solution that contains ammonia and ammonium chloride :
NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH^- (aq) .....(1)
NH₄Cl (aq) → NH₄⁺ (aq) + Cl^- (aq) ..... (2)
One drop of dilute HCl is added into the buffer solution.  What is the effect on the buffer solution once the equilibrium has been re-establish?

• Equilibrium (1) has shifted to the left
• An overall increase in the concentration of NH₄⁺ ions
• An overall increase in the OH^- ion concentration
• An overall increase in the H⁺ ion concentration

A mixture of citric acid and sodium citrate act as a buffer in foodstuff.  Which statement is not true of the reacion in the mixture?

• The dissociation of the citric acid increases in the presence of an alkali
• Citric acid reacts with OH^- ion in the presence of alkali
• Citric ion reacts with H⁺ ions in the presence of acid
• Na⁺ ions react with OH^- in the presence of alkali

The pK_a value for NH₄⁺ ion is 9.2 at 298 K. What is the pK_b value for aqueous ammonia at this temperature?

• 4.8
• 9.2
• 11.8
• 12.0

The pH of a weak monoprotic acid is 4.0 if its concentration is 0.01 mol dm^-3.  What is the pK_a value for this acid?

• 4.0
• 5.0
• 6.0
• 6.5

HIn is an acid-base indicator which is blue in the molecular form and yellow in the ionic form.  The dissociation constant of this indicator is 1.0 x 10^-4 mol dm^-3.  What is the colour of this indicator in a solution with pH 7?

• Yellow
• Purple
• Blue
• Green

The degree of dissociation of an acid, HA of concentration of 0.050 mol dm^-3 is 5%.  The dissociation constant of that acid is .....

• 1.25 x 10^-4 mol dm^-3
• 2.50 x 10^-3 mol dm^-3
• 5.00 x 10^-3 mol dm^-3
• 0.05 mol dm^-3