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General Chemistry I · Gases

Gas Collected over Water

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

When a gas is collected by bubbling it through water into an inverted tube, the collected gas is "wet" — it is mixed with water vapor. The total pressure inside the tube is the sum of the dry gas's pressure and the vapor pressure of water. To find the pressure of the gas alone, you subtract the water-vapor pressure: P_gas = P_total − P_H2O.

Why this matters

Collecting a gas over water is one of the most common ways to measure the amount of gas produced by a reaction in the lab. Forgetting to subtract the water-vapor pressure is a classic source of error — it makes the gas pressure (and therefore the computed moles) too high. The same correction applies to respiratory physiology, where alveolar air is always saturated with water vapor.

The college version

Key Ideas

  • Wet gas: the collected sample contains both the target gas and water vapor.
  • Water-vapor pressure depends only on temperature (e.g., 23.8 mmHg at 25 °C).
  • Dalton's law applies: P_total = P_gas + P_H2O.
  • Correction: P_gas = P_total − P_H2O.
  • Level the water: the water inside and outside the tube is leveled so the total pressure equals the atmospheric (barometric) pressure.

Equations and Variables

  • P_total = P_gas + P_H2O (Dalton's law for the wet sample).
  • P_gas = P_total − P_H2O — the dry-gas pressure.
  • P_total is usually the barometric (atmospheric) pressure once the water levels are equalized.
  • Ideal gas law for the dry gas: n_gas = P_gas·V/(RT).
  • P_H2O is looked up in a vapor-pressure table for the water's temperature.

How It Works

  1. The gas is produced (e.g., by a reaction) and bubbled into a tube that has been filled with water and inverted in a water bath.
  2. The gas displaces the water and collects at the top of the tube.
  3. The gas in the tube is saturated with water vapor, so it is a two-component mixture: target gas + water vapor.
  4. The tube is raised or lowered until the water level inside matches the level outside, making the pressure inside equal to atmospheric pressure.
  5. Subtract the water-vapor pressure (from a table at the bath temperature) to get the dry gas pressure, then use PV = nRT.

Worked Example

Oxygen gas is collected over water at 25 °C, where the vapor pressure of water is 23.8 mmHg. The barometric pressure is 745 mmHg, and the collected volume is 0.500 L. Find the pressure and amount of dry O₂. P(O₂) = P_total − P_H2O = 745 mmHg − 23.8 mmHg = 721 mmHg. Convert to atm: 721 mmHg × (1 atm / 760 mmHg) = 0.949 atm. T = 25 + 273.15 = 298.15 K. n = PV/RT = (0.949 atm)(0.500 L) / [(0.08206 L·atm/mol·K)(298.15 K)] = 0.0194 mol O₂.

Common Confusions

  • "The collected gas is pure" — it's always mixed with water vapor; you must subtract P_H2O.
  • "Water-vapor pressure depends on the gas collected" — it depends only on the water's temperature, not on the gas.
  • "Use the wet (total) pressure in PV = nRT" — that overcounts the moles; use the dry-gas pressure.
  • "P_H2O is negligible" — near room temperature it's ~24 mmHg, about 3% of atmospheric pressure, enough to change a result.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine blowing bubbles through a straw into a glass of water, then trapping them under an upside-down cup. The "gas" you trapped isn't pure — it's your breath plus water that evaporated into the bubbles. To figure out how much of your gas is really there, you have to subtract the little bit of pressure that comes from the water vapor. The analogy's limit: the water-vapor amount is fixed by the water's temperature alone, whereas the amount of your breath depends on how hard you blow — but the "subtract the water's share" idea is exact.

Key takeaways

  • P_gas = P_total − P_H2O.
  • P_H2O depends only on temperature, not on the identity or amount of the collected gas.
  • At 25 °C, P_H2O = 23.8 mmHg.
  • At 100 °C, P_H2O = 760 mmHg (water boils).
  • Dalton's law underlies the correction.
  • Always use the dry gas pressure in PV = nRT for the target gas.
  • Collected-over-water gas is "wet": it contains water vapor.
  • P_gas = P_total − P_H2O.
  • P_H2O is read from a table at the water temperature.
  • Use the dry-gas pressure in the ideal gas law.
  • Dalton's law: total = gas + water vapor.

Keep learning

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Practice General Chemistry I

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Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Explain how a gas is collected over water in the lab.
  • Account for water vapor when calculating the pressure of the collected gas.
  • Correct the "wet" pressure to find the dry gas pressure.
  • Use the corrected pressure in the ideal gas law to find moles.

Sources & references

  1. OpenStax, "9.3 Stoichiometry of Gaseous Substances, Mixtures, and Reactions," Chemistry 2e.
  2. Brown et al., "10.6 Gas Mixtures and Partial Pressures," Chemistry LibreTexts.
  3. NIST Chemistry WebBook.

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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