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A 100 mL sample of a urea solution contains 6.02 × 10²⁰ molecules of urea. What is the molarity of the solution? (Take Avogadro constant, N_A = 6.02 × 10²³ mol^−1.)

1. 0.02 M
2. 0.01 M
3. 0.001 M
4. 0.1 M

Correct answer: 0.01 M

Solution

6.02e20 / 6.02e23 = 1e-3 mol of urea in 0.100 L, so molarity = 0.001/0.1 = 0.01 M. Stored answer 0.001 M confuses moles with molarity.

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