General formulae

2 marks · Easy

The general formula for alkanes is .

An alkane molecule contains 5 carbon atoms.

Calculate the number of hydrogen atoms in this molecule. Show your working.

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

12 (±0)

Also accept: Twelve

Common mistakes: Using the formula for alkenes instead of alkanes (would get 10); Forgetting to use +2 (gives 10 as answer)

Show mark scheme and worked solution

Marking points

M1 · 1 mark · Substitutes n = 5 into the alkane general formula: H = 2n + 2, or writes out the full formula.

A1 · 1 mark · Correctly calculates 2 × 5 + 2 = 12 hydrogen atoms. (follow-through)

Worked solution

The general formula for alkanes is .

Here, (number of carbon atoms).

There are 12 hydrogen atoms in this alkane molecule.