Understanding Gas Collection over Water & Partial Pressure Lab
A eudiometer works because a gas can push water out of an inverted, water-filled tube. The trapped sample reaches equilibrium with the water around it, so some liquid water enters the gas phase as vapor. This happens even when the collected gas came from a reaction and contained no water at the start.
Water vapor pressure is controlled mainly by temperature. Warmer water has faster moving molecules, so more molecules escape the liquid surface and enter the gas space. A vapor pressure table is only valid at its listed temperature, which is why the water bath temperature must be measured carefully before making the correction.
The pressure inside the tube equals atmospheric pressure only when the water level inside the tube matches the level outside. At equal levels, the water on both sides has no height difference to support an extra pressure difference. If the levels are uneven, the higher water column indicates lower gas pressure on that side, and a hydrostatic pressure correction is needed before using the result.
The dry gas pressure is found by removing the pressure due to water vapor from the total pressure of the mixture. This step is based on the idea that each gas particle contributes to the total pressure independently through collisions with the container walls. Forgetting this correction makes the dry gas pressure too large, which then makes the calculated amount of gas too large.
The ideal gas law connects the corrected pressure with the measured volume and the bath temperature in kelvin. Pressure units must match the gas constant being used, and volume is commonly converted to liters before calculation. Students should watch for air bubbles present before collection, gas that dissolves in water, leaks at the stopper, and a tube that was not fully filled with water, since each problem changes the amount of gas measured.
