Gce Model Questions Vol1 2019 Chemistry Question 14

Adipic acid (\(\mathrm{C_6H_{10}O_4}\), \(M = 146.14\ \text{g mol}^{-1}\)) and hexamethylenediamine (\(\mathrm{C_6H_{16}N_2}\), \(M = 116.21\ \text{g mol}^{-1}\)) undergo step‑growth condensation polymerisation to give a polyamide, releasing one molecule of water (\(M_{\mathrm{H_2O}} = 18.02\ \text{g mol}^{-1}\)) per repeat unit. A sample containing \(10.0\ \text{g}\) of adipic acid also contains \(2.0\%\) by weight of a monofunctional impurity that can react at only one end of a chain. The diamine is present in large excess. The reaction proceeds to \(80\%\) conversion of the functional acid groups. (a) Calculate the number‑average degree of polymerisation (\(DP_n\)) taking the impurity into account. (b) Using the repeat‑unit mass \(M_{\text{repeat}} = M_{\text{acid}} + M_{\text{amine}} - M_{\mathrm{H_2O}}\), calculate the weight‑average molecular weight (\(M_w\)) of the polymer formed.