4.5 Acid-base Equilibria | Edexcel International A Level Chemistry Topic Questions 2017

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Hard

8 marks

This question is about compounds with the molecular formula C₆H₁₂O₂.

Hexanoic acid, C₅H₁₁COOH, is a weak acid.

Write the equation for the acid dissociation constant, K_a, of hexanoic acid.

(1)

ii)

Calculate the pH of a 0.100 mol dm^–3 solution of hexanoic acid.
[pK_a of hexanoic acid = 4.88]

(4)

iii)

The solubility of hexanoic acid in water is 1.08 g per 100 g of water.
The isomer of hexanoic acid, butyl ethanoate, CH₃CO₂C₄H₉, has a solubility of 0.68 g per 100 g of water.

Explain the differences in these data in terms of the hydrogen bonding between hexanoic acid and water, and between butyl ethanoate and water.

(3)

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10 marks

Compound A is thought to be another isomer of hexanoic acid.

10 g of compound A is found to contain 6.21 g of carbon and 1.03 g of hydrogen, with the remainder being oxygen.

Use the data to calculate the empirical formula of compound A.

You must show all your working.

(3)

ii)

State how you might use your answer to (b)(i) and a mass spectrum of compound A to prove that compound A is an isomer of hexanoic acid.

(1)

iii)

A series of tests was performed on compound A.

	Test	Observation
1	addition of phosphorus(V) chloride	misty fumes
2	addition of 2,4-dinitrophenylhydrazine	orange precipitate
3	addition of Benedict’s or Fehling’s reagent	solution remains blue with no precipitate
4	addition of acidified potassium dichromate(VI)	solution remains orange
5	test using polarimetry	plane of plane polarised light is rotated

Deduce the structure for compound A.
Justify your answer by using all the test results.

(6)

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1c2 marks

Compound B, another isomer with the molecular formula C₆H₁₂O₂, contains a ring of six carbon atoms.

The carbon-13 NMR spectrum has only two peaks, one of which is at 69 ppm.

Draw the structure of compound B.

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8 marks

Sodium hydrogensulfate is a widely used acid, with applications that include removing limescale and as a food additive. Sodium hydrogensulfate is a weak acid because of the presence of the hydrogensulfate ion, HS $O_{4}^{-}$ .

Write the equation for the dissociation of the hydrogensulfate ion in aqueous solution. State symbols are not required.

(1)

ii)

A solution of sodium hydrogensulfate has pH = 1.13
Calculate the concentration of this solution, in g dm⁻³.
[pK_a of HS

O_{4}^{-}

= 1.92]

(5)

iii)

State the assumptions you have used in (a)(ii).

(2)

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9 marks

A solution containing sodium hydrogensulfate and sodium sulfate is a buffer that is used to preserve urine for steroid analysis.

State what is meant by the term buffer.

(2)

ii)

Calculate the pH of the buffer prepared by dissolving 0.750 mol of sodium hydrogensulfate and 0.500 mol of sodium sulfate in distilled water to make 1.00 dm³ of solution.

(3)

iii)

Separate samples of 0.00500 mol of hydrochloric acid are added to 1.00 dm³of distilled water and to the buffer in (b)(ii).

Calculate the pH changes that result in each case.
Assume that the volumes remain constant at 1.00 dm³.

(4)

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2c3 marks

The titration curve obtained from the addition of sodium hydroxide solution to a weak acid is shown. The equivalence point (E) of this titration occurred at pH = 8

q20c-paper-4-oct-2021-edexcel-ial-chemistry

Explain the observations that would be made if methyl orange (pK_In = 3.7) were used as the indicator for this titration.

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Acid-base Equilibria (Edexcel International A Level Chemistry)

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