A blend contains 10 g of polymer of molar mass $10000 g mo l^{- 1}$ (one species) and 10 g of polymer of molar mass $40000 g mo l^{- 1}$ (one species). Use $M_{n} = \sum N_{i} M_{i} / \sum N_{i}$ . The number-average molar mass ( $g mo l^{- 1}$ ) is

Answer: A

Moles:

10/10000 = 1.0 \times 1 0^{- 3}

and

10/40000 = 2.5 \times 1 0^{- 4}

M_{n} = 20/ (1.25 \times 1 0^{- 3}) = 16000

. The simple mean of the two masses,

25000

, ignores that the lighter chains are more numerous (here it also equals the mass-weighted average

M_{w}

, since the two masses are equal).