Chemistry / Chem 1219 · Procedure · 60–90 seconds
Calculating Gas Pressure from a Manometer Reading
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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.
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.
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.