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PCAT Chemical Processes – Gas Behavior

A 2 L sealed vessel contains 2 moles of nitrogen gas at 0oC. All of the following will decrease the pressure inside the vessel EXCEPT:

  1. nitrogen leaking out of the tank
  2. decreasing the temperature of the gas
  3. expanding the volume to 3 L
  4. replacing 1 mole of nitrogen gas for 1 mole of oxygen gas

This question is testing your understanding of gas behavior according to the ideal gas law.

PV = nRT

When nitrogen leaks out of the tank, the moles of nitrogen, n, decreases. Since pressure is proportional to moles, decreasing the moles of gas will decrease the pressure making A incorrect. Since temperature is also proportional to pressure, a decrease in temperature will lower the pressure making B false. Pressure and volume are inversely related, which means that increasing the volume will result in a decrease in pressure. Therefore, C is not the correct answer, which leaves D as the correct answer. Exchanging 1 mole of nitrogen gas for 1 mole of oxygen changes the partial pressures within the system but has no effect on total pressure.

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