Chemistry question help pls

Hi, please could I have some help on question1fi of this paper, I’m not sure how the markscheme shows two repeats, it looks like three repeats to me.
Question: https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/CAIE/Paper-4/QP/November%202016%20(v1)%20QP.pdf
Markscheme: https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/CAIE/Paper-4/MS/November%202016%20(v1)%20MS.pdf
Thanks!

Reply 1

JQ2neeky

Original post by anonymous56754

ethane-1,2-diamine has molecular formula H2NCH2CH2NH2. Writing out ethanedioic acid looks like this - HOOC-COOH. If we combine them to make a polymer, One hydrogen leaves each NH2 group on the diamine and a OH leaves both end of the acid. So your left with HNCHCHNH and OC-CO. putting them together we get HNCHCHNHOCCO. as a repeating unit. Obviously you repeat that once.

You can tell that what they have in the mark scheme is 2 repeats because if we look at the repeating unit we worked out, we had 2 oxygens (or C=O) but now we have 4 (so that suggests it's double). the same goes for the NH groups.

Hope that helped 🙂

Reply 2

anonymous56754

Original post by JQ2neeky

ethane-1,2-diamine has molecular formula H2NCH2CH2NH2. Writing out ethanedioic acid looks like this - HOOC-COOH. If we combine them to make a polymer, One hydrogen leaves each NH2 group on the diamine and a OH leaves both end of the acid. So your left with HNCHCHNH and OC-CO. putting them together we get HNCHCHNHOCCO. as a repeating unit. Obviously you repeat that once.
You can tell that what they have in the mark scheme is 2 repeats because if we look at the repeating unit we worked out, we had 2 oxygens (or C=O) but now we have 4 (so that suggests it's double). the same goes for the NH groups.
Hope that helped 🙂

Thank you!