IBDP Official Exam Material
FREE Past Paper Questions by Topic
If you need the markscheme, including the examiners' reports for each question, and more exam and teaching materials on this topic, contact me through the contact page
Topic 18: Acids and Bases
For HL students only
1. [1 mark]
Which solutions will form a buffer when mixed?
A. 50 cm3 of 1.0 mol dm−3 HCl and 50 cm3 of 1.0 mol dm−3 NaOH
B. 50 cm3 of 1.0 mol dm−3 CH3COOH and 50 cm3 of 1.0 mol dm−3 NaOH
C. 50 cm3 of 1.0 mol dm−3 CH3COOH and 100 cm3 of 1.0 mol dm−3 NaOH
D. 100 cm3 of 1.0 mol dm−3 CH3COOH and 50 cm3 of 1.0 mol dm−3 NaOH
2. [1 mark]
Which species can act both as a Lewis acid and a Lewis base?
A. H2O
B. NH4+
C. Cu2+
D. CH4
3. [1 mark]
Which statement explains the Lewis acid–base nature of the chloride ion in this reaction?
C2H5+ + Cl– → C2H5Cl
A. Lewis base because it donates a pair of electrons
B. Lewis base because it accepts a pair of electrons
C. Lewis acid because it donates a pair of electrons
D. Lewis acid because it accepts a pair of electrons
4. [1 mark]
In which set are the salts arranged in order of increasing pH?
A. HCOONH4 < KBr < NH4Br < HCOOK
B. KBr < NH4Br < HCOOK < HCOONH4
C. NH4Br < HCOONH4 < KBr < HCOOK
D. HCOOK < KBr < HCOONH4 < NH4Br
5. [1 mark]
A weak base is titrated with a strong acid. Which value of pKb can be estimated from this titration curve?
6. [1 mark]
Which species are both Lewis and Brønsted–Lowry bases?
I. CN−
II. OH−
III. NH3
II. OH−
III. NH3
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
7a. [1 mark]
Ammonia is soluble in water and forms an alkaline solution:
NH3 (g) + H2O (l) ⇌ NH4+ (aq) + HO– (aq)
State the relationship between NH4+ and NH3 in terms of the Brønsted–Lowry theory.
7b. [2 marks]
Determine the concentration, in mol dm–3, of the solution formed when 900.0 dm3 of NH3 (g) at 300.0 K and 100.0 kPa, is dissolved in water to form 2.00 dm3 of solution. Use sections 1 and 2 of the data booklet.
7c. [1 mark]
Calculate the concentration of hydroxide ions in an ammonia solution with pH = 9.3. Use sections 1 and 2 of the data booklet.
7d. [2 marks]
Calculate the concentration, in mol dm–3, of ammonia molecules in the solution with pH = 9.3. Use section 21 of the data booklet.
7e. [2 marks]
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) ⇌ 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.
7f. [2 marks]
Magnesium salts form slightly acidic solutions owing to equilibria such as:
Mg2+ (aq) + H2O (l) ⇌ Mg(OH)+ (aq) + H+ (aq)
Comment on the role of Mg2+ in forming the Mg(OH)+ ion, in acid-base terms.
7g. [2 marks]
Mg(OH)+ is a complex ion, but Mg is not regarded as a transition metal. Contrast Mg with manganese, Mn, in terms of one characteristic chemical property of transition metals, other than complex ion formation.
8a. [1 mark]
The overall equation for the production of hydrogen cyanide, HCN, is shown below.
CH4 (g) + NH3 (g) +32O2 (g) → HCN (g) + 3H2O (g)
State why NH3 is a Lewis base.
8b. [3 marks]
Calculate the pH of a 1.00 × 10−2 mol dm−3 aqueous solution of ammonia.
pKb = 4.75 at 298 K.
8c. [1 mark]
Justify whether a 1.0 dm3 solution made from 0.10 mol NH3 and 0.20 mol HCl will form a buffer solution.
9a. [2 marks]
50.00 cm3 of 0.75 mol dm−3 sodium hydroxide was added in 1.00 cm3 portions to 22.50 cm3 of 0.50 mol dm−3 chloroethanoic acid.
Calculate the initial pH before any sodium hydroxide was added, using section 21 of the data booklet.
9b. [1 mark]
The concentration of excess sodium hydroxide was 0.362 mol dm−3. Calculate the pH of the solution at the end of the experiment.
9c. [3 marks]
Sketch the neutralisation curve obtained and label the equivalence point.
10a. [1 mark]
Phosphoric acid, H3PO4 can form three different salts depending on the extent of neutralisation by sodium hydroxide.
Formulate an equation for the reaction of one mole of phosphoric acid with one mole of sodium hydroxide.
10b. [2 marks]
Formulate two equations to show the amphiprotic nature of H2PO4−.
10c. [2 marks]
Calculate the concentration of H3PO4 if 25.00 cm3 is completely neutralised by the addition of 28.40 cm3 of 0.5000 mol dm−3 NaOH.
11. [1 mark]
Which compound is acidic in aqueous solution?
A. KBr
B. CH3COONa
C. NH4Cl
D. Na2CO3
12. [1 mark]
What is the order of increasing pH for the following solutions of the same concentration?
A. NaCl < NH4Cl < Na2CO3 < CH3COONa
B. CH3COONa < NH4Cl < NaCl < Na2CO3
C. NH4Cl < NaCl < CH3COONa < Na2CO3
D. Na2CO3 < CH3COONa < NaCl < NH4Cl
13. [1 mark]
Which species is not a Lewis base?
A. OH−
B. NH4+
C. H2O
D. PH3
14. [1 mark]
An indicator, HIn, has a pKa of 5.1.
HIn (aq) ⇌ 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−]
B. At pH = 3, colour B would be observed
C. At pH = 7, [HIn] = [In−]
D. At pH = 3, [HIn] < [In−]
15. [2 marks]
Butanoic acid, CH3CH2CH2COOH, is a weak acid and ethylamine, CH3CH2NH2, is a weak base.
State the equation for the reaction of each substance with water.
Butanoic acid:................................................................................................................................................
Ethylamine:................................................................................................................................................
If you need the markscheme, including the examiners' reports for each question, and more exam and teaching materials on this topic, contact me through the contact page