Ethene ( $C_{2} H_{4}$ ) polymerises to polyethylene according to the simplified equation $n\ \mathrm{C_2H_4} \rightarrow \text{(C_2H_4)}_n$ . If $5.0 L$ of ethene at STP is completely polymerised, calculate the mass of polyethylene formed (molar mass of the repeating unit $\mathrm{C_2H_4} = 28\ \text{g mol^{-1}}$ ). Assume the polymer contains the same number of repeating units as moles of ethene reacted.

medium 3 marks Class 10 · Science · Carbon and its Compounds

Answer

6.25 g of polyethylene

The correct answer is 6.25 g of polyethylene — State the final mass.

Step-by-step solution

At STP, 1 mol of any gas occupies $22.4 L$ .
Moles of ethene = $\frac{5.0 L}{22.4 L mol ^{- 1}} = 0.2232 mol$ .
Each mole of ethene becomes one mole of the repeating unit in the polymer, so moles of repeating units = 0.2232 mol.
Mass of polymer = moles $\times$ molar mass of the repeating unit = $0.2232 mol \times 28 g mol^{- 1} = 6.25 g$ .
State the final mass.

Common mistake: Using the molar volume of liquids instead of gases, or forgetting that the mass of polymer equals the combined mass of the monomer units.

Concept tested: Polymerisation and molar calculations

Published 25 August 2026 · Last updated 16 September 2026 · How this solution was prepared and checked

Step-by-step solution

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