Question

100 POINTS PLS HELP MUST SHOW WORK

4. . A common laboratory technique is to collect gas from a chemical reaction by what is called water
displacement. In water displacement, the volume of the gas collected is equal to the volume of
water that is displaced from the container. Unfortunately, the gas collected is not entirely limited
to the gas released from the reaction. Because of the presence of water, a small amount of water
vapor contaminates the gas sample. What is the pressure of the gas produced in the reaction if the
atmospheric pressure at 25°C is 775 mmHg?

Answer 1

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

In order to solve a problem, it is necessary to know the vapor pressure of water at the temperature of the reaction (see Table below). The sample problem illustrates the use of Dalton’s law when a gas is collected over water.

Vapor Pressure of Water (mmHg) at Selected Temperatures (°C)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

4.58 6.54 9.21 12.79 17.54 23.76 31.82 42.18 55.32 71.88 92.51 118.04 149.38 187.54 233.7

Sample Problem: Gas Collected by Water Displacement

A certain experiment generates 2.58 L of hydrogen gas, which is collected over water. The temperature is 20°C and the atmospheric pressure is 98.60 kPa. Find the volume that the dry hydrogen would occupy at STP.

Step 1: List the known quantities and plan the problem.

Known

Unknown

The atmospheric pressure is converted from kPa to mmHg in order to match units with the table. The sum of the pressures of the hydrogen and the water vapor is equal to the atmospheric pressure. The pressure of the hydrogen is found by subtraction. Then, the volume of the gas at STP can be calculated by using the combined gas law.

Step 2: Solve.

Now the combined gas law is used, solving for

, the volume of hydrogen at STP.

Step 3: Think about your result.

If the hydrogen gas were to be collected at STP and without the presence of the water vapor, its volume would be 2.28 L. This is less than the actual collected volume because some of that is water vapor. The conversion using STP is useful for stoichiometry purposes.

Summary

The vapor pressure due to water in a sample can be corrected for in order to get the true value for the pressure of the gas.

Review

Why is gas collected over water not pure?

Why would we want to correct for water vapor?

In a lab, 2.20 L of gas is collected over water at a temperature of 30°C and a total pressure of 735.43 mmHg. Find the volume that the dry nitrogen of gas would occupy at STP.

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