CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 14. The Behavior of Gases

Practice Problem

1. A mixture of 0.249 mol He, 0.812 mol Ne, and 0.429 mol Ar are mixed into a container at a total pressure of
   1.340 atm. Calculate the partial pressure of each gas.

Collecting a Gas by Water Displacement

Gases that are produced in laboratory experiments are often collected by a technique called water displacement (
Figure14.14). A bottle is filled with water and placed upside-down in a pan of water. The reaction flask is fitted
with rubber tubing that is fed under the bottle of water. As the gas is produced in the reaction flask, it exits through
the rubber tubing and displaces the water in the bottle. When the bottle is full of the gas, it can be sealed with a lid.

Note that if the water level inside the bottle is the same as the water level outside the bottle, the pressure of the
gas in the bottle must be equal to the ambient atmospheric pressure outside the bottle. If the levels are different,
the difference in height can be used to determine the difference in pressure between the enclosed sample and the
external atmosphere. In this book, we will assume that the bottle is adjusted so that the inner and outer water levels
are equal.

FIGURE 14.14

A gas produced in a chemical reaction

can be collected by water displacement.

However, the collected gas will not be pure. Because it is collected over water, the gas will be mixed with water
vapor due to evaporation. Dalton’s law can be used to calculate the amount of the desired gas by subtracting the
contribution of the water vapor.

PTotal=P

g + P_H_2O

P

gisthe pressureo f thedesiredgas.

P

g = P_Total - P_H_2O

In order to solve a problem, it is necessary to know the vapor pressure of water at the temperature of the collection
vessel (Table14.4). Sample Problem 14.9 illustrates the use of Dalton’s law when a gas is collected over water.

TABLE14.4: Vapor Pressure of Water (mmHg) at Selected Temperatures (°C)

Temperature (°C) Vapor Pressure (mmHg) Temperature (°C) Vapor Pressure (mmHg)

0 4.58 40 55.32

5 6.54 45 71.88

10 9.21 50 92.51