Collecting a Gas Over Water

Applications of Dalton's Law: Collecting a gas over water

Let's say you want to generate H₂(g) by reacting zinc metal with hydrochloric acid

Zn(s) + 2HCl(aq) ---> ZnCl₂(aq) + H₂(g)

We can set up an apparatus to collect the gas.

When you collect a gas over water, you fill a test tube with water and invert it under the water so that the entire tube stays filled with water when it is held upright. The gas you are collecting is bubbled into the test tube and the gas displaces the water.

Here the pressure of the gases inside the test tube is less than atmospheric pressure.

When the level of the water inside the test tube equals the level of the water in the container, then

P_atmospheric = P_{inside the test tube}

You can adjust the level of the gas inside the test tube with the level of the water in the contaner so that the pressure of the gas inside the test tube is equal to the atmospheric pressure.

If the gas level inside the test tube is more than the level of the water in the container, then the pressure inside the test tube is greater than atmosphereic pressure.

When a gas is collected over water, inside the test tube there will be some water vapor. What you collect is a mixture of the gas you produced and water vapor. The total pressure in the test tube is due to the partial pressure of the gas collected and the partial pressure of the water vapor.

P_total = P_gas + P_H2O

You can measure P_tot and can look up in a table the vapor pressure of water, P_wataer at the temperature of the water in the experiment.

Temp., °C	Vapor pressure of water, mmHg
5.0	6.543
10.0	9.209
15.0	12.79
20.0	17.54
35.0	42.18
50.0	92.15
80.0	355.1
100.0	760.0

The vapor pressure of water increases as the temperature of the water increases. At 100°C, the vapor pressure of water equals atmosphereic pressure.

If we collected 310.0 mL of gas over water at a pressure of 738 mmHg and a temperature of 20.0°C, what is the partial pressure of H₂(g) in the test tube?

P_total = P_H2 + P_H2O

P_total - P_H2O = P_H2

738 mmHg - 17.54 mmHg = 721 mm Hg = P_H2

721 mm Hg x 1.00 atm/760.0 mmHg = 0.949 atm = P_H2