Equal masses (50 g each) of two monodisperse fractions of molar mass $10000$ and $40000 g mo l^{- 1}$ are mixed. The number-average molar mass in $g mo l^{- 1}$ is

Answer: B

M_{n}

= total mass/total moles

= 100/ (50/10000 + 50/40000) = 100/ (0.00500 + 0.00125) = 16000

. The value

25000

M_{w}

(mass-weighted mean).