Alkanes
3 marks · NormalOctane (C8H18) can undergo substitution with chlorine in the presence of UV light.
Write the balanced chemical equation for the mono-substitution reaction of octane with chlorine. Give the formulae for all reactants and products. Calculate the total number of hydrogen atoms remaining in all the organic product molecules if 5 moles of octane completely undergo mono-substitution.
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
C8H18 + Cl2 → C8H17Cl + HCl; 5 moles of octane yield 5 moles of C8H17Cl; each C8H17Cl has 17 hydrogens; 5 × 17 = 85 hydrogen atoms in total in the organic product molecules. hydrogen atoms per molecule × 5 molecules
Common mistakes: Using the incorrect general formula; Not subtracting one hydrogen per substituted molecule
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Writes the correct balanced equation: C8H18 + Cl2 → C8H17Cl + HCl.
M2 · 1 mark · Recognises each substituted octane molecule has 17 hydrogens after substitution.
A1 · 1 mark · Calculates the total number of hydrogen atoms in 5 moles = 5 × 17 = 85 hydrogens.
Worked solution
The substitution reaction is:
Mono-substitution replaces one H with one Cl, so C8H17Cl is formed with 17 H atoms per molecule.
5 moles of octane will form 5 moles of C8H17Cl:
So there are 85 hydrogen atoms in the 5 organic product molecules.