Question 23M.2.HL.TZ1.1
| Date | May 2023 | Marks available | [Maximum mark: 14] | Reference code | 23M.2.HL.TZ1.1 |
| Level | HL | Paper | 2 | Time zone | TZ1 |
| Command term | Calculate, Deduce, Determine, Estimate, Explain, Identify, Outline | Question number | 1 | Adapted from | N/A |
This question is about acid–base properties.
Deduce the ionic equation, including state symbols, for the reaction of hydrogen chloride gas with water.
[2]
H2O (l) + HCl (g) → Cl− (aq) + H3O+ (aq) ✓✓
One for the equation and one for the state symbols.
Do not accept H2O (l) + H+(g)→ H3O+(aq)
Do not accept equilibrium sign.
Calculate the pH of 0.50 mol dm−3 hydrochloric acid.
[1]
«» ✓
Explain why a solution of ethanoic acid has a higher pH than hydrochloric acid of the same concentration.
[1]
«Ethanoic acid» partially ionizes/dissociates/OWTTE
OR
lower [H+] ✓
Do not accept weak acid only.
Accept converse argument.
A pH probe can be used to distinguish between the acids in part (a)(iii). Identify another simple instrumental method that could be used in a school laboratory to distinguish between the two acids.
[1]
conductivity/conductance meter/probe
OR
ammeter «with power supply» ✓
Ignore any reference to indicators or any chemical methods.
Accept Cl− or ethanoate ion selective probe.
Outline how the instrumental method identified in part (a)(iv) distinguishes between the acids in part (a)(iii).
[1]
HCl higher conductivity «due to higher [ion]» ✓
Accept explanation if alternative given in a(iv).
Accept converse argument.
Apply ECF for incorrect method.
Outline one chemical test, other than an indicator, that can distinguish between the two acids in part (a)(iii), and the expected result.
Chemical test:
Expected result:
[1]
Chemical test:
use of carbonate/hydrogen carbonate/named metal
AND
Expected result:
more bubbles per unit time/disappears faster/faster reaction in HCl(aq) ✓
OR
Chemical test:
add alkali/hydroxide/metal oxide
AND
Expected result:
higher temperature rise with HCl ✓
OR
Chemical test:
add silver nitrate «solution»/AgNO3«(aq)»
AND
Expected result:
white precipitate/ppt. with HCl ✓
Do not accept just metal.
Accept active metal.
Accept greater temperature change in place of more bubbles.
A neutralization curve for a weak acid, HA, and a strong base is given.
Estimate the pKa of HA.
[1]
4.8 ✓
Accept 4.7–4.9
Explain, using an equation, why adding a strong base to the weak acid, HA, leads to very little change in pH in the buffer zone of the graph.
[2]
ALTERNATIVE 1
HA + OH– ⇌ A– + H2O ✓
added OH– neutralized by HA
OR
strong base «OH–» replaced by weak base «A–» ✓
ALTERNATIVE 2
HA A– + H+ ✓
added OH– neutralized by H+
OR
strong base «OH–» replaced by weak base «A–» ✓
Must show ⇌ for M1
Accept molecular equation.
Allow reference to Châtelier principle for M2
In a separate experiment, 80 cm3 of 0.1 mol dm−3 ammonia, NH3 (aq), was added to 40 cm3 of 0.1 mol dm−3 hydrochloric acid, HCl (aq).
Determine the final pH of the solution. Use section 21 of the data booklet.
[4]
n(NH3)init = «0.08 dm3 × 0.1 mol dm–3 =» 0.008 mol
AND
n(HCl)init = «0.04 dm3 × 0.1 mol dm–3 =» 0.004 mol ✓
n(NH3)fin = «0.008 mol − 0.004 mol =» 0.004 mol
AND
n(NH4+)fin = 0.004 mol ✓
«Vfin = 0.08 dm3 + 0.04 dm3 = 0.12 dm3»
«c(NH3)fin = c(NH4+)fin = 0.004 mol/0.12 dm3 = 0.033 mol dm–3»
pKa(NH4+) = «14 – pKb(NH3) = 14 – 4.75 =» 9.25 ✓
pH = «9.25 + log(0.033/0.033) =» 9.25
OR
pH = «9.25 + log(0.004/0.004) =» 9.25 ✓
Award [4] for the correct final answer.
Accept alternate working.
