Multiple choice

A flask containing 12 g of a gas of molecular mass 120 g at a pressure of 100 atm was evacuated by means of a pump until the pressure was 0.01 atm. Which of the following is the best estimate of the number of molecules left in the flask?

  1. 2 × 1018

  2. 3 × 1018

  3. 4 × 1018

  4. 6 × 1018

  5. 6×1023
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

 P1V = n1RT              (i) P2V = n2RT                                                    (ii) (n2/n1 ) = (P2/P1)  n2 = (P2×n1) / P1 = (12/120) × (0.01/100) = 10-5 Since 1 mole contains 6.023 × 1023 molecules, 10-5 mole contains molecules = 10-5 × 6.023 × 1023 = 6.023 × 1018 (Approx. 6 × 1018)

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