A large polythene molecule is found to have a relative molecular mass of 4.0 x 10^4 - VCE - SSCE Chemistry - Question 17 - 2002 - Paper 1

Polythene (polyethylene) is a polymer made up of repeating units of ethylene, which has a formula of C₂H₄.

1. First, find the molar mass of one repeat unit (C₂H₄):

   • Carbon (C) has an atomic mass of approximately 12 g/mol, so:
     • Mass of 2 Carbon atoms = 2 × 12 = 24 g/mol
   • Hydrogen (H) has an atomic mass of approximately 1 g/mol, so:
     • Mass of 4 Hydrogen atoms = 4 × 1 = 4 g/mol
   • Total molar mass of C₂H₄ = 24 + 4 = 28 g/mol.

2. Next, to find the number of repeat units in the large polythene molecule:

   • The relative molecular mass is given as 4.0 x 10^4 g/mol.
   • Number of repeat units = (4.0 x 10^4 g/mol) / (28 g/mol)
   • Number of repeat units ≈ 1428.57
   • Therefore, we can round this to approximately 1429 repeat units.

3. Since each repeat unit contains 2 carbon atoms:

   • Total number of carbon atoms = 1429 × 2 = 2858.

4. Comparing with the options provided, the closest answer is B: 2900.