Biodegradable polyesters

3 marks · Normal

Lactic acid has the molecular formula . A single lactic acid molecule is used as the monomer for making polylactic acid (PLA).

Calculate the molecular formula of the repeat unit of PLA, given that one molecule of water () is eliminated each time two lactic acid monomers link. Show your working.

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C3H4O2

Also accept: C3H4O2

Common mistakes: Forgetting to subtract H2O; Incorrect subtraction of atoms

Show mark scheme and worked solution

Marking points

M1 · 1 mark · States correct molecular formula for lactic acid monomer (C3H6O3).

M2 · 1 mark · Subtracts H2O (i.e., loses 2 H and 1 O) for formation of the repeat unit.

A1 · 1 mark · Arrives at correct repeat unit formula: C3H4O2. (follow-through)

Worked solution

Lactic acid: C3H6O3.

In forming the repeat unit by condensation polymerisation, one H2O is lost per linkage.

C3H6O3 − H2O = C3H4O2 (subtract 2 H, 1 O).

Molecular formula of PLA repeat unit: C3H4O2.