Making a polyester

3 marks · Normal

A polyester is formed from 10 molecules of butane-1,4-diol (HOCH2CH2CH2CH2OH) and 10 molecules of hexane-1,6-dioic acid (HOOCCH2CH2CH2CH2CH2COOH). Calculate the number of water molecules produced when these monomers react completely to form one long polyester chain.

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19 (±0)

Also accept: 19 (from 10 diol and 10 dicarboxylic acid molecules)

Common mistakes: Saying 20 (number of monomers); Not subtracting 1 from (2n)

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Marking points

M1 · 1 mark · Recognises correct relationship: (2n – 1) water molecules produced for n of each monomer.

M2 · 1 mark · Substitutes n = 10 into the formula (2 × 10 – 1). (follow-through)

A1 · 1 mark · Correct answer: 19 water molecules. (follow-through)

Worked solution

Number of water molecules = (2n – 1) where n is the number of each monomer.

So, 19 water molecules are produced.