Unsaturated hydrocarbons

3 marks · Normal

A straight-chain alkene contains 7 carbon atoms. Calculate:

(a)

Write the molecular formula of this alkene.

[1 mark]

(b)

Calculate the total number of hydrogen atoms in 5 molecules of this alkene.

[2 marks]

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(a) C7H14

(b) 70

Common mistakes: Using alkane formula: C7H16; Multiplying C7H14 incorrectly

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Marking points

B1 (a) · 1 mark · Correctly gives C7H14 as the molecular formula for a straight-chain alkene with 7 carbons.

M1 (b) · 1 mark · Multiplies 14 hydrogens by 5 molecules or equivalent correct working.

A1 (b) · 1 mark · Gives the correct final answer: 70 hydrogens. (follow-through)

Worked solution

a) The general formula for alkenes is CnH2n. For n = 7, the molecular formula is C7H14.

b) Each molecule has 14 hydrogens. For 5 molecules: 14 × 5 = 70 hydrogens.