Chemistry / Unit 7 Chem 1279 worked example, 60 to 90 seconds
Correcting for Water Vapor Pressure
The idea
Correcting a gas-over-water measurement means looking up the vapor pressure of water at the collection temperature from a reference table, then subtracting that value from the measured total pressure to isolate the dry gas's own pressure.
In practice
Argon collected over water at 26 °C shows a total pressure of 750 torr. The vapor pressure of water at 26 °C, read from a standard reference table, is 25.2 torr, so subtracting gives a dry argon pressure of 724.8 torr, which converts to about 0.954 atm for use in any later moles-of-argon calculation.
Where people slip
Using the vapor pressure listed for a different temperature than the one actually measured — the value changes quickly with temperature — corrupts the dry-gas result and any mole or volume calculation built on top of it.
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