O-LevelChemistryOrganic Chemistry5070/21 · Oct/Nov 2015 · Q910 Marks

The structure of glycolic acid can be represented as shown. [Figure B9.1: Structural diagram of glycolic acid showing HO–CH2–COOH.]

(a)Glycolic acid is a solid at room temperature. Describe the arrangement and motion of the molecules in glycolic acid at room temperature. arrangement motion[2]
(b)(i)Glycolic acid can polymerise with itself to form a polyester called poly(glycolic acid). What type of polymer is a polyester?[1]
(b)(ii)Draw a section of the polymer chain of poly(glycolic acid) showing at least two repeating units.[2]
(c)(i)Glycolic acid is produced by heating methanal, carbon monoxide and water in the presence of a sulfuric acid catalyst. CH2O + CO + H₂O → CH2(OH)COOH methanal glycolic acid A sample of 1800g of methanal reacts with excess carbon monoxide and water. The percentage yield of glycolic acid is 45%. Calculate the mass, in grams, of glycolic acid produced. mass of glycolic acid =[3]
(c)(ii)Glycolic acid is a weak acid. Sulfuric acid is a strong acid. Explain the difference between a strong acid and a weak acid.[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2015). The mark or mark type is shown in brackets.

[2] B9 (a) arrangement: regularly arranged/in a set pattern/ordered/not random/fixed position (1) motion: vibrating/do not move (from place to place) (1)
[1] (b) (i) condensation (polymer) (1)
[2] (ii) correct structure with minimum of two units (2) e.g. i -O-□-C-O - □ – C – (as minimum required)
[3] (c) (i) moles methanal (= 1800/30) = 60 mol (1) mass of glycolic acid (= 60 × 76) = 4560 (g) (1) for 45% yield (= 4560 × 45/100) = 2052 (g) (1)
[2] (ii) strong acid is fully ionised/fully dissociated in solution (1) weak acid is partially ionised/incompletely dissociated in solution (1)

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