Question

How does the pressure of the H₂ gas (P_gas) collected in the 500 mL graduated cylinder compare with the pressure of the air (P_air) in the laboratory?

Select one:

a. P_gas = P_air

b. P_gas < P_air due to vapor pressure of water and height difference of water in cylinder and trough.

c. P_gas > P_air due to vapor pressure of water and height difference of water in cylinder and trough.

d. P_gas < P_air due to vapor pressure of water

e. P_gas > P_air due to vapor pressure of water

Answer 1

Answer: d. P_gas < P_air due to vapor pressure of water

Vapor pressure is the pressure exerted on the inside of a container due to the vapors that escape from a liquid. Some particles in a liquid have enough energy to go into the gas phase. Since the particles are in gas phase, they exert a pressure on the inside of the container, but they can also go back into liquid phase so an equilibrium is reached where a constant pressure due to the vapors is assumed. This pressure changes with temperature because at higher temperatures, the liquid has more energy, and likewise there is a greater probability of the gas escaping, and the escaped gas has more energy so it exerts a greater force on the container. Therefore, the vapor pressure is directly proportional to temperature. The height difference between the gas level in the graduated cylinder and the water level in the beaker is thus a source of error. The pressure inside the cylinder is slightly greater than atmospheric pressure because the trapped gas is supporting the extra height of the column of water (assuming the water level in the beaker is above the gas level in the graduated cylinder). The pressure difference may be accounted for by measuring the height (h) in millimeters of the water level above the gas level in the graduated cylinder.

P_atm = P_air + P_H2O

or P_air = P_gas + P_H2O

P_cylinder = P_atm + h (mm H₂O) × (1 mm Hg/13.6 mm H₂O)

According to Dalton’s Law of Partial Pressure, the total pressure of a mixture of gases is equal to the sum of the partial pressures of each gas present. In this experiment, therefore, the total pressure of the gas is the same as the pressure within the room (barometric pressure, when the water level inside the cylinder = water level in the beaker), is the sum of the pressure of the water vapor plus the pressure of the hydrogen gas.

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