IB Questionbank

User interface language: English | Español

Date	November 2021	Marks available	1	Reference code	21N.2.hl.TZ0.6
Level	HL	Paper	2	Time zone	TZ0
Command term	Suggest	Question number	6	Adapted from	N/A

Question

Biochemical oxygen demand (BOD) can be determined by the Winkler Method.

A 25.00 cm³ sample of water was treated according to the Winkler Method.

Step I: 2Mn²⁺(aq) + O₂(g) + 4OH⁻(aq) → 2MnO₂(s) + 2H₂O (l)

Step II: MnO₂(s) + 2I⁻(aq) + 4H⁺(aq) → Mn²⁺(aq) + I₂(aq) + 2H₂O (l)

Step III: 2S₂O₃²⁻(aq) + I₂(aq) → 2I⁻(aq) + S₄O₆²⁻(aq)

The iodine produced was titrated with 37.50 cm³ of 5.000 × 10⁻⁴mol dm⁻³ Na₂S₂O₃.

Outline what is measured by BOD.

[1]

A student dissolved 0.1240 ± 0.0001 g of Na₂S₂O₃ to make 1000.0 ± 0.4 cm³ of solution to use in the Winkler Method.

Determine the percentage uncertainty in the molar concentration.

[2]

Calculate the amount, in moles of Na₂S₂O₃ used in the titration.

[1]

c(i).

Deduce the mole ratio of O₂ consumed in step I to S₂O₃²⁻ used in step III.

[1]

c(ii).

Calculate the concentration of dissolved oxygen, in mol dm⁻³, in the sample.

[2]

c(iii).

The three steps of the Winkler Method are redox reactions.

Deduce the reduction half-equation for step II.

[1]

c(iv).

Suggest a reason that the Winkler Method used to measure biochemical oxygen demand (BOD) must be done at constant temperature.

[1]

c(v).

Markscheme

«amount of» oxygen used to decompose the organic matter in water ✔

« $\frac{0.0001 g}{0.1240 g} \times 100 % =$ » 0.08 «%»
OR
« $\frac{0.4 {cm}^{3}}{1000.0 {cm}^{3}} \times 100 % =$ » 0.04 «%» ✔

«0.08 % + 0.04 % =» 0.12/0.1 «%» ✔

Award [2] for correct final answer.

Accept fractional uncertainties for M1, i.e., 0.0008 OR 0.0004.

« $\frac{37.50 {cm}^{3}}{1000}$ × 5.000 × 10⁻⁴mol dm⁻³ =» 1.875 × 10⁻⁵«mol» ✔

c(i).

1:4 ✔

Accept “4 mol S₂O₃^2– :1 mol O₂“, but not just 4:1.

c(ii).

« $1.875 \times 10^{- 5} mol \times \frac{1}{4} =$ » 4.688 × 10⁻⁶«mol» ✔

« $\frac{4.688 \times 10^{- 6} mol}{\frac{25.00 {cm}^{3}}{1000}} =$ » 1.875 × 10⁻⁴«mol dm⁻³» ✔

Award [2] for correct final answer.

c(iii).

MnO₂(s) + 2e⁻ + 4H⁺(aq) → Mn²⁺(aq) + 2H₂O (l) ✔

c(iv).

rate of reaction of oxygen with impurities depends on temperature
OR
rate at which bacteria/organisms grow/respire depends on temperature ✔

c(v).

Examiners report

[N/A]

c(i).

[N/A]

c(ii).

[N/A]

c(iii).

[N/A]

c(iv).

[N/A]

c(v).

Syllabus sections

Core » Topic 4: Chemical bonding and structure » 4.4 Intermolecular forces

Show 79 related questions

Hide related questions

Core » Topic 4: Chemical bonding and structure

Core

Question

Markscheme

Examiners report

Syllabus sections

View options