A sample has 2 mol of chains of molar mass $20000 g mo l^{- 1}$ and 1 mol of chains of molar mass $50000 g mo l^{- 1}$ . Using $M_{n} = \sum N_{i} M_{i} / \sum N_{i}$ , $M_{w} = \sum N_{i} M_{i}^{2} / \sum N_{i} M_{i}$ and PDI $= M_{w} / M_{n}$ , the PDI is

Answer: B

M_{n} = (2 \times 20000 + 50000) /3 = 30000

M_{w} = (2 \times 2000 0^{2} + 5000 0^{2}) / (2 \times 20000 + 50000) = 36667

. PDI

= 36667/30000 = 1.22