Unit 5: Gas Chemistry (5.5) Solving Gas Pressure Problems

If the problem involves using a closed-arm manometer, the answer is the difference in the mercury level converted to kilopacals.

Conversion 1 kPa = mm of Hg

If the problem involves an open-arm manometer, first convert the difference in the mercury level to kilopascals, then either add or subtract the difference to the given atmospheric pressure.

If the mercury is higher in the arm connected to the gas, then the pressure of the mercury must be subtracted from the atmospheric pressure. If the mercury is higher in the open arm, then the pressure of the mercury must be added to the atmospheric pressure.

Remember: The greater the height of the liquid in one arm of a manometer is a measure of the greater pressure of the gas in the opposite arm.

Examples of determining the gas pressure involving each type of manometer. In the open-arm manometer, the mercury level is higher in the arm connected to the gas; so the pressure of the gas is less than that of the atmosphere. Therefore, the difference in the mercury level must be subtracted from the atmospheric pressure. (Do note the omission error involving the open- arm manometer. The equation 55 mm = 87.3 kPa should have been 655 mm = 87.3 kPa.)