Question

A polymer sample contains 10 molecules each with molecular mass 5,000 and 5 molecules each with molecular mass 50,000. The number average molecular mass of the polymer sample is

• A. \(2 \times 10^4\)
• B. \(3 \times 10^4\)
• C. \(2 \times 10^5\)
• D. \(3 \times 10^5\)

Hint:

Number average molecular mass is weighted average based on number of molecules.

Answer 1

The Correct Option is A

Number average molecular mass is given by: \[ M_n = \frac{\sum n_i M_i}{\sum n_i} \] Where \(n_i\) is number of molecules and \(M_i\) is molecular mass. \[ M_n = \frac{(10 \times 5000) + (5 \times 50000)}{10 + 5} = \frac{50000 + 250000}{15} = \frac{300000}{15} = 20000 = 2 \times 10^4 \]