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ASChemistryEdexcelExam Questions1. Physical Chemistry1.7 Reaction Calculations

Reaction Calculations (Edexcel AS Chemistry)

Exam Questions

2 hours11 questions
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.

i)
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.

i)
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.

i)
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  

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1d
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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 (✓)        

i)
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)

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1e
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The teacher carried out the experiment and obtained the following results.

Mass of sodium hydroxide used to make 250.0 cm3  solution = 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

  1. 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.
  2. When the reaction in Step 1 was finished, more metal carbonate was added until the metal carbonate was in excess.
  3. The resulting mixture was filtered into an evaporating basin.
  4. The evaporating basin was heated using a Bunsen burner to concentrate the solution. The concentrated solution was allowed to cool and crystallise.
  5. Once crystal formation was complete, the resulting mixture was filtered for a second time.
  6. 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)

i)
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)

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2b
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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. 

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2c
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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. 

i)
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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3a1 mark

Malachite is a green mineral with the formula Cu2CO3(OH)2 . It has a molar mass of 221 g mol–1.

What is the percentage by mass of copper in pure malachite?

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

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3b3 marks

Describe what you would expect to see when an excess of dilute hydrochloric acid is added to a sample of pure solid malachite.

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3c7 marks
i)
Describe how you would carry out a flame test on a sample of powdered malachite.
(3)

ii)
When the atoms of some elements are heated, they produce a characteristic flame colour. For example, the copper in malachite gives a blue-green colour. Explain how atoms of different elements can produce different characteristic flame colours when heated.
(4)

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3d
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i)
When malachite is heated to approximately 300 °C, water, carbon dioxide and copper(II) oxide are formed.

The equation for this decomposition is

Cu2CO3(OH)2 → 2CuO + CO2 + H2O

Calculate the maximum volume of carbon dioxide that could be produced when 0.810 g of malachite is thermally decomposed.

Assume that the gas is collected at a temperature of 25 °C and 101 kPa pressure.

Give your answer to an appropriate number of significant figures and state the units.[The ideal gas equation is pV = nRT. Gas constant (R) = 8.31 J mol–1K–1]

(5)

ii)
The gas was collected in a gas syringe with a stated accuracy of ±0.5 cm3.

Calculate the percentage uncertainty in the volume of gas collected.

(1)

iii)
Malachite ore is a mixture of malachite and rock. A 0.810g sample of malachite ore was thermally decomposed, producing 0.571g of copper(II) oxide.

Calculate the percentage purity of this malachite ore sample.
Give your answer to an appropriate number of significant figures.

(3)

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4a
Sme Calculator10 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 cm3 sample 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

i)
Name the piece of apparatus used to measure 25.0 cm3 of 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)

v)

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)

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4b
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The ethanol content of alcoholic drinks is usually measured as the percentage of alcohol by volume (ABV).

ABV = begin mathsize 16px style fraction numerator Volume space of space ethanol space in space a space solution over denominator Total space volume space of space the space solution end fraction end style× 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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