1.7 Reaction Calculations

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Hard

1a3 marks

This question is about a titration experiment carried out by a group of students to determine the concentration of a solution of ethanoic acid using sodium hydroxide.

A student weighed about 4.00 g of sodium hydroxide pellets and added them to a beaker containing 50 cm³ of deionised water.

The mixture was stirred with a glass rod to dissolve the pellets and to give a homogenous solution.

The solution was poured through a funnel into a 250.0 cm³ volumetric flask and deionised water was added up to the mark and then the flask was shaken.

Describe how you would ensure that all the sodium hydroxide was transferred to the volumetric flask.

(2)

ii)

A student adds deionised water above the mark and shakes the flask.

State why the procedure has to be restarted rather than using a teat pipette to remove the excess water.

(1)

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1b3 marks

Two students each cleaned a burette, then poured sodium hydroxide solution into their burettes.

Student 1 used a funnel to pour sodium hydroxide solution into the burette.

Give two steps needed before the student takes the initial burette reading.

(2)

ii)

Student 2 cleaned the burette by rinsing it with deionised water immediately before filling it with the sodium hydroxide solution.

Give the effect, if any, on the value of the first titre. Justify your answer.

(1)

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1c5 marks

The sketch shows the pH changes during a titration of 25.0 cm³ of ethanoic acid with sodium hydroxide of the same concentration.

q3c-9cho-al-3-nov-2020-qp-edexcel-a-level-chem

The ideal indicator for this titration will change colour on the addition of a very small volume of sodium hydroxide solution at a titre value very close to the equivalence point of the reaction.

Assess the suitability of methyl red as an indicator for this titration. Make use of the Data Booklet in answering this question.

(4)

ii)

Complete the table, with a tick (✓) or a cross (✗), to show whether or not the indicator would be suitable for use in this titration.

(1)

Indicator	pH range	Tick or Cross
Bromocresol purple	5.2 – 6.8
Thymol blue	8.0 – 9.6
Thymolphthalein	8.3 – 10.6
Alizarin yellow R	10.1 – 13.0

How did you do?

2 marks

Each student used a pipette to measure 25.0 cm³ of the ethanoic acid solution into four separate conical flasks and added an indicator.

The results of one student’s titrations are shown in the table.

Titration number	1	2	3	4
Final burette reading / cm³	13.00	25.50	37.90	50.00
Initial burette reading / cm³	0.25	13.00	25.50	37.90
Titre / cm³
Concordant titres (✓)

Complete the table.

(1)

ii)

The low titre for titration 4 was queried by the teacher. The student had wanted to refill the burette and continue the titration but had been told the measurement uncertainty would increase if this was done.

Calculate the total percentage measurement uncertainty if the burette had been refilled to 0.00, and then a further 0.30 cm³ had been added from the burette, to the conical flask.

The measurement uncertainty for each burette reading is ±0.05 cm³.

(1)

How did you do?

5 marks

The teacher carried out the experiment and obtained the following results.

Mass of sodium hydroxide used to make 250.0 cm³solution = 3.80 g

Volume of ethanoic acid solution = 25.00 cm³

Mean titre of sodium hydroxide = 11.90 cm³

CH₃COOH (aq) + NaOH (aq) → CH₃COONa (aq) + H₂O (l)

Calculate the concentration of the ethanoic acid solution in g dm⁻³.

Give your answer to an appropriate number of significant figures.

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2a5 marks

A student made crystals of a metal chloride, JCl₂.6H₂O, by reacting the metal carbonate, JCO₃ , with hydrochloric acid, HCl(aq). The product was purified.

Procedure

150 cm³ of hydrochloric acid, concentration 0.80 mol dm⁻³, was transferred to a 400 cm³ conical flask. The flask was warmed gently using a Bunsen burner. A spatula measure (about 1.0 g) of metal carbonate was added to the acid.
When the reaction in Step 1 was finished, more metal carbonate was added until the metal carbonate was in excess.
The resulting mixture was filtered into an evaporating basin.
The evaporating basin was heated using a Bunsen burner to concentrate the solution. The concentrated solution was allowed to cool and crystallise.
Once crystal formation was complete, the resulting mixture was filtered for a second time.
The resulting white crystals were rinsed with a small volume of ice-cold water.

The equation for the reaction between the metal carbonate and hydrochloric acid is

JCO₃ (s) + 2HCl (aq) → JCl₂ (aq) + H₂O (l) + CO₂ (g)

Describe two observations that the student might make which show that the reaction in Step 1 has finished.

(2)

ii)

State the purpose of the filtration in Step 3.

(1)

iii)

Explain the use of a small volume of ice-cold water in Step 6.

(2)

How did you do?

5 marks

The student obtained a mass of 14.26 g of hydrated crystals.

Assuming that the percentage yield is 100%, use the information in the procedure to give a possible identity of J.

How did you do?

5 marks

The student was surprised by the white colour of the crystals of JCl₂.6H₂O in Step 6.

This did not agree with the possible identity for J from the calculation in (b).

The student decided to perform a flame test on the crystals.

Explain why the student was surprised and decided to carry out a flame test.

(2)

ii)

The flame test colour was crimson red. Identify J.

(1)

☐	A	barium
☐	B	calcium
☐	C	lithium
☐	D	strontium

iii)

Calculate the actual percentage yield of the reaction, which produced 14.26 g of crystals.

Give your answer to two significant figures.

(2)

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10 marks

Wine and gin are aqueous solutions of ethanol with traces of other organic
compounds which give these drinks their characteristic flavours and aromas.

When a bottle of wine is opened, oxidation of the ethanol in the wine produces ethanoic acid.

An experiment was carried out to determine the percentage of the ethanol that had been oxidised.

A bottle of white wine, with an ethanol concentration of 2.50 mol dm^–3,was opened and left to stand at room temperature for three weeks.
A 25.0 cm³sample of the wine was transferred to a conical flask and phenolphthalein indicator added.
Aqueous sodium hydroxide of concentration 0.235 mol dm^–3 was added from a burette until the colour of the indicator permanently changed.
The titration was repeated and the titre values, in cm³, were 27.90, 26.75 and 26.85.

The equation for the neutralisation reaction is

CH₃COOH + NaOH → CH₃COO^–Na+ + H₂O

Name the piece of apparatus used to measure 25.0 cm³of wine.

(1)

ii)

To improve the accuracy, the burette should be rinsed.

State what should be used.

(1)

iii)

What is the colour change at the end-point?

(1)

☐	A	from orange to yellow
☐	B	from red to orange
☐	C	from colourless to pink
☐	D	from pink to colourless

iv)

State what is meant by the term ‘concordant results’ as applied to a titration experiment.

(1)

Calculate the percentage of ethanol that has oxidised, given that one mole of ethanol forms one mole of ethanoic acid.

Give your answer to an appropriate number of significant figures.

(5)

vi)

Deduce why this method would not be effective for the analysis of the acid content of a red wine.

(1)

How did you do?

3 marks

The ethanol content of alcoholic drinks is usually measured as the percentage of alcohol by volume (ABV).

ABV = $\frac{Volume of ethanol in a solution}{Total volume of the solution}$ × 100

The ABV values of four different brands of gin are shown.

Brand of gin	ABV (%)
A	40
B	42
C	44
D	46

A sample of one of these gins was found to contain an ethanol concentration of 7.50 mol dm^–3.

By calculating the percentage of ethanol by volume (ABV) of this sample, deduce the brand of this gin.

[Assume the density of ethanol, C₂H₅OH = 0.79 g cm^–3]

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Edexcel A Level Chemistry

Topic Questions

1.7 Reaction Calculations

☐	A	40.3%
☐	B	51.4%
☐	C	57.5%
☐	D	67.9%