# Chemistry Questions and Answers – Molecular Mass of Polymers

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This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Molecular Mass of Polymers”.

1. Molecular mass of polymers are expressed as a/an _______
a) average
b) median
c) mode
d) percentage

Explanation: The length of polymer and their molecular mass depends on the number of monomers available for reaction. As a result, the chain lengths of polymers are varying and are expressed as an average.

2. Polydispersity index is defined as ______ where Mw and Mn are the weight average and number average molecular masses respectively.
a) Mw x Mn
b) Mw/Mn
c) Mn/Mw
d) Mw – Mn

Explanation: Polydispersity index (PDI) is defines as the ratio of weight average molecular mass to the number average molecular mass. It gives an idea of the homogeneity of the polymer.

3. The polydispersity index of natural polymers is _________
a) 0
b) <0.8
c) 1
d) >1.2

Explanation: Natural polymers generally have chains of identical lengths and have definite molecular masses (Mw=Mn). This makes them more homogeneous and monodisperse, with a PDI of approximately 1.
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4. Calculate the number average molecular mass of a polymer having four different monomers A, B, C and D present in equal number. The molecular masses of the monomers are 10000, 15000, 30000 and 50000 respectively.
a) 10050
b) 17350
c) 26250
d) 35475

Explanation: Since the 4 monomers are present in equal amount, there is 25% of each monomer in the polymer. The number average molecular mass is,
$$\Rightarrow$$ Mn = (25×10000 + 25×15000 + 25×30000 + 25×50000)/(25+25+25+25)
$$\Rightarrow$$ Mn = (250000+375000+750000+1250000)/100
$$\Rightarrow$$ Mn = 2625000/100
$$\Rightarrow$$ Mn = 26250.

5. The polydispersity index can be less than 1.
a) True
b) False

Explanation: The weighted average molecular mass is always greater than the number average molecular mass. This is because of the 2-degree nature of Mw (because of squaring of masses). Hence PDI ≥ 1.

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