This question was popular and well attempted by candidates. In (a) candidates were able to list characteristics of homologous series. They were also able to identify the compound CH3(CH2)2CH3 and gave its isomers.
In (b), majority of the candidates were able to identify the products and write down their correct structure. In (d), candidates were able to give the IUPAC names of the compounds. However some candidates could not state a chemical test for each of the functional groups mentioned.
In (e) majority of the candidates were able to calculate the empirical and molecular formula of the compound.
The expected answers include:
Question 8
(a) (i) - they can be represented by a general molecular formula;
- they have similar chemical properties;
- they have same functional group;
- they can be prepared by a general method;
- each consecutive member differ by –CH2 group/14 amu;
- increase intensity of physical properties with increase in molar mass;
I - Alkanes
H H H H
II - H – C – C – C – C – H
H H H H
H
H H C H H
II H – C C – C – C – H
H H H H
(ii) III butane
2-methylpropane
(b) (i) H
O
H – C – C – OH
H
(ii) H H
H – C – C – Cl
H H
(iii) H H H H
O
II - H – C – C – C – O – C – C – H
H H H H
(c) (i) ethanoic acid
(ii) chloroethane
(iii) ethylpropanoate
(d) (i) X is 2-methylpropan-2-ol
Y is phenylmethanoic acid/benzoic acid
(ii) To sample X add some PCl5 . Steaming white fumes of HCl will be observed
OR
Add an organic acid to sample X and few drops of concentrated H2SO4 or dry HCl gas and
heat. A pleasant/ fragrance/fruity smell is produced, confirms –OH in X
Add (saturated solution) of NaHCO3/NaCO3 to sample Y. Effervescence of a colourless,
odourless gas which turns lime water milky
OR
To sample Y, add an alkanol in the presence of conc. H2SO4 or dry HCl.
A pleasant/fragrance/fruity smell is produced (1), confirms –COOH in Y
(e) (i) C H O
moles =
moles =
4 4 1
empirical formula= C4H4O
(ii) (C4H4O)n = 136
68n = 136
n =
n= 2
molecular formula – C8H8O2
