Multiple choice

A sample of polymer contains $30\%$ molecules with molecular mass $20,000$, $40\%$ with molecular mass $30,000$ and $30\%$ with molecular mass $60,000$. What is the number average molecular mass of the polymer?

  1. $27,000$
  2. $36,000$
  3. $43,000$
  4. $64,000$
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B Correct answer
Explanation

The number average molecular mass is the weighted average of the molecular masses: (0.3 * 20000) + (0.4 * 30000) + (0.3 * 60000) = 6000 + 12000 + 18000 = 36000.

AI explanation

To find the number average molecular mass, calculate the weighted sum of the molecular masses based on their percentages and divide by the total percentage. The calculation is (30 percent times 20,000) plus (40 percent times 30,000) plus (30 percent times 60,000). This equals 6,000 plus 12,000 plus 18,000, resulting in a total of 36,000. Since the total percentage accounts for one whole, the average is 36,000 divided by 1, yielding a final mass of 36,000.

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