O-LevelChemistryOrganic Chemistry5070/22 · Oct/Nov 2019 · Q610 Marks

Butanoic acid, C3H7COOH, is a carboxylic acid.

(a)Draw the structure of butanoic acid to show all of the atoms and all of the bonds.[1]
(b)Describe the movement and arrangement of the particles in liquid butanoic acid. movement arrangement[2]
(c)The melting point of butanoic acid is –4°C. The boiling point of butanoic acid is 166°C. Deduce the physical state of butanoic acid at 0°C. Explain your answer. physical state: explanation:[1]
(d)The equation for the reaction of butanoic acid with sodium carbonate is shown. 2C3H7COOH + Na2CO3 → 2C3H7COONa + CO2 + H2O A student added 5.28g of butanoic acid to 56.0 cm³ of 0.500 mol/dm³ sodium carbonate. Show by calculation that butanoic acid is in excess.[3]
(e)Magnesium reacts with butanoic acid. The products are magnesium butanoate and hydrogen. Construct the equation for this reaction.[1]
(f)One method of determining the pH of aqueous butanoic acid is by using a pH meter. Describe a different method of determining the pH of aqueous butanoic acid.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[1] correct structure of butanoic acid showing all atoms and all bonds [H-C-C-C-C-O-H with H's filling bonds, and C=O on the rightmost C and OH]
[1] movement: sliding (over each other)
[1] arrangement: random / no fixed positions
[1] liquid AND 0 °C is above the melting point and below the boiling point / 0°C is between the melting and boiling points
[1] mol butanoic acid = 5.28 / 88 OR 0.06 mol
[1] mol sodium carbonate = 56 / 1000 × 0.500 OR 0.028
[1] (butanoic acid in excess because sodium carbonate × 2) = 0.056 mol OR (butanoic acid in excess because butanoic acid ÷ 2) = 0.03 mol
[1] Mg + 2C3H7COOH → (C3H7COO)2Mg + H2
[1] use universal indicator / full range indicator
[1] match the colour observed with a colour chart

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