Chemistry / Unit 7 Chem 1276 short lesson, about 20 seconds
Collecting a Gas Over Water
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The idea
A gas collected by bubbling it into an inverted, water-filled container becomes mixed with water vapor, so the trapped gas's total pressure includes both.
In practice
Hydrogen collected over water at a measured total pressure is mixed with whatever water vapor pressure applies at that temperature.
Where people slip
The measured total pressure overstates the collected gas's own pressure until the water vapor's contribution is subtracted out.
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