Introduction to Organic Chemistry
5 marks · EasyThe molecule pentane belongs to a homologous series of organic compounds.
State what is meant by a 'homologous series' in organic chemistry.
Give the general formula for the homologous series that pentane belongs to.
Write the molecular formula for pentane.
Draw the displayed formula for pentane.
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.
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(a) A series of organic compounds with the same functional group, similar chemical properties, and each member differing by a –CH2– unit.
(b) CnH2n+2
(c) C5H12
(d) — · must show 5 carbon atoms in a straight chain, each with enough hydrogen atoms to make 4 bonds for C, 1 for H; all bonds shown
Common mistakes: Naming the wrong general formula; Omitting required bonds in displayed formula
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Defines a homologous series as a set of organic compounds with the same functional group, similar properties, and each differing by –CH2–.
B1 (b) · 1 mark · States the correct general formula: CnH2n+2 for alkanes.
B1 (c) · 1 mark · Gives correct molecular formula for pentane: C5H12.
B1 (d) · 2 marks · Displayed formula: All C and H atoms shown, all bonds included, correct number of each. Award 1 for correct chain, 1 for all bonds correctly drawn.
Worked solution
a) A homologous series is a family of organic compounds with the same functional group, similar chemical properties, and each member differs from the next by a –CH2– group.
b) The general formula for alkanes (the series pentane belongs to) is CnH2n+2.
c) Pentane has 5 carbon atoms. Using the formula: n = 5, so the formula is C5H12.
d) Displayed formula:
Each carbon is attached to enough H atoms to make 4 bonds in total. Draw all single bonds between atoms.