This is the most popular question among the candidates and the performance was good .
In (a)(i) and (ii), candidates correctly wrote the two possible structural isomers of C5H8O and gave the class of the isomers as any alkanol/alkoxyalkane/ether.
In(b)(i), most candidates named the substance and the conditions necessary for fermentation as yeast and 250C – 400C for 2-4 days respectively.
In (b)(ii) I and II, candidates correctly stated CO2 as the gas produced and described that the gas if passed into lime water would turn milky.
In(b) (iii), majority of the candidates correctly gave the names of substances Q, R and S as polythene/polyethylene, 1, 2 – dibromoethane and ethane respectively.
In (b) (iv), only a few of the candidates could correctly write the balanced equation for the reaction between ethene and bromine. The expected answer from candidates was
C2H4 + Br2 → C2H4Br2
In (c)(i) – (iv), most candidates were able to correctly name the compound as propane, gave two reagents from either bromine water or KMnO4 or K2Cr2 O7 solution, wrote an equation for the reaction between bromine and the compound but could not successfully calculate the mass of bromine required to completely react with 7.0g of the compound.
The required solution to (iv) was:
From the equation
42g of compound reacts with 160g of Br2
∴ 7g of compound = 7 x 160
42
=26.67g
In (d), majority of the candidates correctly determined the molecular formula of alaxin thus:
(C5H8 )nO5 = 284
( 12 x 5 + 8 x 1 ) n + 16 x 5 = 284
68n = 284 – 80
n = 204
68
n = 3
C15H24O5
