Chemistry / Unit 7 Chem 1219 worked example, 60 to 90 seconds
Calculating Gas Pressure from a Manometer Reading
The idea
Calculating a gas's pressure from a manometer means measuring the height difference between the two liquid columns, converting that height to a pressure unit, then adding it to atmospheric pressure for an open-end manometer or reading it directly for a closed-end manometer.
In practice
An open-end mercury manometer shows the gas-side column 13.7 cm lower than the open side. Converting 13.7 cm to 137 mm Hg and adding standard atmospheric pressure, 760 mm Hg, gives a gas pressure of 897 mm Hg, because the gas is pushing harder than the atmosphere, so its column sits lower than the open arm's.
Where people slip
Adding atmospheric pressure when the gas-side column actually sits higher — meaning the gas pressure is lower than atmospheric — reverses the sign and produces a pressure that is too high instead of too low.
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