Gas collected over water is mixed with water vapor, so its pressure equals total minus vapor pressure.
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When a gas is collected by displacing water, the collected gas is saturated with water vapour and is therefore a mixture. The pressure of the dry gas equals the total atmospheric pressure minus the vapour pressure of water at that temperature, P_gas = P_atm - P_water. The vapour pressure of water depends strongly on temperature, so the correction must be looked up from a table. Once the dry gas pressure is known, the ideal gas law can be applied to calculate moles or molar mass. Ignoring this correction is a common source of error in experiments that measure gas volumes.
Key ideas: Steps: read the barometric pressure, look up the water vapour pressure at the collection temperature, subtract to get the dry gas pressure, then use PV = nRT. Water vapour pressure is about 17.5 mmHg at 20 C, 23.8 at 25 C and 31.8 at 30 C. The water levels inside and outside the tube should be equalized before reading.
Common mistakes and connections: Forgetting the subtraction inflates the moles of gas collected. Another error is using a vapour pressure value for the wrong temperature. The method fails for gases that dissolve readily in water, like ammonia or hydrogen chloride. It combines Dalton's law, vapour pressure and the ideal gas law, and is used in determining molar mass of magnesium reacting with acid.
A gas collected over water is mixed with water vapour; subtract the water vapour pressure at that temperature to get the pressure of the dry gas. The interactive lesson shows this as a live animation or chart.
At 25°C the vapour pressure of water is 23.8 mmHg, so a gas collected at 760 mmHg total pressure has a dry gas pressure of about 736 mmHg.
Gas collected over water is mixed with water vapor, so its pressure equals total minus vapor pressure.
Gas Collection Over Water is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in States of Matter & Solutions first: Absolute Zero, Dalton's Law of Partial Pressures, Mole Fraction.
The next topics in States of Matter & Solutions are Graham's Law of Effusion and Diffusion.
Yes. The interactive lesson includes a self-check question on Gas Collection Over Water, and the States of Matter & Solutions quiz draws on every topic in this area.
Gas Collection Over Water is topic 18 of 109 in States of Matter & Solutions.