Introduction to Organic Chemistry
5 marks · EasyA hydrocarbon in the alkane series contains 7 carbon atoms.
Write the general formula for alkanes.
Calculate the molecular formula for this alkane, showing your working.
Draw the structural (condensed) formula for this alkane.
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) CnH2n+2
(b) C7H16; working: H atoms = (2×7)+2 = 16, so formula is C7H16.
(c) — · must show Condensed formula: CH3CH2CH2CH2CH2CH2CH3 (or equivalent)
Common mistakes: Calculating H using n instead of 2n+2
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States general formula: CnH2n+2.
M1 (b) · 1 mark · Correctly substitutes n = 7 into formula.
A1 (b) · 1 mark · Correct molecular formula: C7H16.
B1 (c) · 2 marks · Condensed structural formula: all 7 C written as chain, hydrogens grouped as CH3 (ends)/CH2 (middle). Both ends as CH3, correct number of groups.
Worked solution
a) General formula for alkanes is CnH2n+2.
b) For n = 7, H = (2×7)+2 = 16; so the formula is C7H16.
c) Condensed structural formula: CH3CH2CH2CH2CH2CH2CH3 OR CH3(CH2)5CH3