General formulae
5 marks · HardTable 1 shows three members of a homologous series.
| Name | Molecular formula |
|---|---|
| Methanoic acid | CH2O2 |
| Ethanoic acid | C2H4O2 |
| Propanoic acid | C3H6O2 |
State the general formula for this homologous series.
Name the functional group found in all members of this homologous series.
Give one feature, other than the general formula, that all members of a homologous series share.
Explain why the compounds in this series have similar chemical properties.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) CnH2nO2
(b) carboxyl group (–COOH)
(c) Same functional group / similar chemical properties / successive members differ by –CH2–.
(d) They have similar chemical properties because they all contain the same functional group (–COOH), which is responsible for their reactions.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States the general formula: CnH2nO2.
B2 (b) · 1 mark · Names the functional group as carboxyl group (–COOH).
B3 (c) · 1 mark · Gives a correct feature of a homologous series (other than formula): same functional group or chemical properties or differ by –CH2–.
E1 (d) · 1 mark · Explains similarity of properties due to same functional group.
E2 (d) · 1 mark · States that the functional group is responsible for chemical reactions.
Worked solution
a) The general formula is CnH2nO2.
b) The functional group is the carboxyl group (–COOH).
c) All members have the same functional group / show similar chemical properties / differ by –CH2– group.
d) The compounds in this series have similar chemical properties because they all have the same functional group (–COOH), which determines the reactions they can undergo.