Alkanes

4 marks · Normal

The table below shows the number of carbon and hydrogen atoms in three different unbranched alkanes.

(a)

Using the data, deduce the general formula connecting the number of hydrogen atoms (H) to the number of carbon atoms (C) in alkanes.

[2 marks]

(b)

Predict the number of hydrogen atoms in a straight-chain alkane with 7 carbon atoms.

[1 mark]

(c)

Name the alkane with 9 carbon atoms.

[1 mark]

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) H = 2C + 2 (or CnH2n+2)

(b) 16

Also accept: For part a: H = 2C + 2 or CnH2n+2

Common mistakes: Using the wrong formula; Incorrect number for part b (should be 16)

Show mark scheme and worked solution

Marking points

B1 (a) · 2 marks · States or deduces the correct general formula: H = 2C + 2, or CnH2n+2.

B2 (b) · 1 mark · Correct prediction for alkane with 7 carbons: 16 hydrogens.

B3 (c) · 1 mark · Gives correct name: nonane.

Worked solution

(a) Looking at the data:

Methane: 1C, 4H (4 = 2×1 +2); Butane: 4C, 10H (10 = 2×4 +2); Nonane: 9C, 20H (20 = 2×9 +2).

The general relationship is H = 2C + 2 (or general formula CnH2n+2).

(b) For 7 carbons: H = 2×7 + 2 = 16.