Gas over water correction: The method used to calculate the amount of dry gas from a measured volume of moist gas (collected over water) is based on which gas law?

Difficulty: Easy

Correct Answer: Dalton’s law of partial pressures

Explanation:


Introduction / Context:
When a gas sample is collected over water, the measured pressure includes contributions from both the dry gas and water vapor. Accurately determining the dry gas volume requires correcting for the water vapor’s partial pressure. This is a staple calculation in physical chemistry and gas analysis.


Given Data / Assumptions:

  • Total pressure P_total = P_dry gas + P_water vapor.
  • Temperature is known so that the saturation vapor pressure of water can be obtained.
  • Ideal-gas behavior is assumed for the correction.


Concept / Approach:
Dalton’s law states that the total pressure of a gas mixture equals the sum of the partial pressures of its components. Thus, P_dry gas = P_total − P_water(vapor). Once P_dry gas is known, use the ideal gas relation to compute moles or corrected volume of the dry gas at the same conditions.


Step-by-Step Solution:

Measure P_total and gas volume at given T.Look up P_sat,water at that T.Compute P_dry = P_total − P_sat,water.Use P_dry * V / (R * T) to obtain moles of dry gas (or correct V accordingly).


Verification / Alternative check:
If the collection temperature is lowered, P_sat,water decreases, making the water-vapor correction smaller—consistent with Dalton’s framework and observed behavior.


Why Other Options Are Wrong:

  • Charles’s and Boyle’s laws relate pairs of state variables but not directly to mixture partial pressures.
  • Avogadro’s hypothesis concerns equal volumes at equal T and P containing equal numbers of molecules.
  • Graham’s law is about diffusion/effusion rates, not pressure correction.


Common Pitfalls:
Forgetting to subtract P_sat,water or using the wrong temperature for P_sat lookup will skew the dry-gas calculation.


Final Answer:
Dalton’s law of partial pressures

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion