A linear polyester has a carboxyl group at each end of every chain. A $2.0 g$ sample needs $4.0 mL$ of $0.100 M$ NaOH for complete neutralisation of the carboxyl groups. Its number-average molar mass in $g mo l^{- 1}$ is

Answer: B

Moles of

- COOH

= 4.0 \times 0.100 = 0.400

mmol. Two carboxyl groups per chain gives

0.200

mmol of chains.

M_{n} = 2.0 g / (0.200 \times 1 0^{- 3} mol) = 10000 g mo l^{- 1}

. One COOH per chain would wrongly give

5000