Unit 5, Topic 5:

Dalton's Law of Partial Pressures (12.2)

Lesson Instructions

Review the learning goals and success criteria for Topic 5: Dalton's Law of Partial Pressures.
- Review the minds on, action, and consolidation sections of the lesson, completing any tasks as outlined.
- Complete the fill-in-the-blanks handouts that have been provided to you. They have been linked below.
- Practice and apply what you've learned by completing the practice tasks outlined at the end of the lesson.

Learning Goals

I am learning to ...

- use Dalton’s Law to find the partial pressure of a gas.
- use Dalton’s Law of Partial pressures to find the pressure of a dry gas when collected over water.

Minds On

In day-to-day life, we measure gas pressure when we use a barometer to check the atmospheric pressure outside or a tire gauge to measure the pressure in a bike tube. When we do this, we are measuring a macroscopic physical property of a large number of gas molecules that are invisible to the naked eye. On the molecular level, the pressure we are measuring comes from the force of individual gas molecules colliding with other objects, such as the walls of their container.

Let's take a closer look at pressure from a molecular perspective and learn how Dalton's Law helps us calculate total and partial pressures for mixtures of gases.

Action

Dalton's Law of Partial Pressures

The most naturally occurring gas samples are mixtures of gases.

e.g., Below is a diagram showing the major components of air.

John Dalton discovered that when there is a mixture of gases, each gas exerts a certain pressure.

Dalton's Law of Partial Pressures

the total pressure of a mixture of NON-REACTING gases is equal to the sum of the partial pressures of the individual gases:

P_Total= P₁ + P₂ + P₃ + … etc.

Example

A mixture of gases is composed of nitrogen, with a partial pressure of 300 kPa, oxygen, with a partial pressure of 600 kPa, and carbon dioxide, with a partial pressure of 450 kPa. What is the total pressure of the mixture?

Example

The atmosphere is primarily composed of nitrogen and oxygen. If the pressure in the atmosphere is 760 mm Hg and the partial pressure of oxygen is 160 mm Hg, what percentage of the atmosphere is oxygen?

Collecting Gases Over Water

Often when experiments are done with gases, the gas must be collected over water.
A large gas collecting tube (called a eudiometer) is filled with water and a reaction will occur in it.
As the reaction occurs, the gas produced will displace water out of the hole in the stopper at the bottom of the eudiometer.
The gas that is collected is a mixture of the gas formed and water vapour.
According to Dalton’s Law of Partial Pressures, the pressure of this mixture of gases is equal to the partial pressure of the gas collected and the partial pressure of the water vapour.

P_total = P_{gas collected} + P_H2O

P_total = the atmospheric pressure of the room

P_H2O= partial Pressure due to water vapour

P_{gas collected} = P_atm – P_H2O

To read the volume of gas collected over water, the eudiometer must be moved up or down until the level of water inside and outside are level (this equalizes the pressure inside and outside the tube).

Let's see this in action!

Example

80.0 L of oxygen is collected over water at 24.0 ⁰C. The atmospheric pressure in the room is 96.00 kPa. What is the partial pressure of the oxygen gas collected?

Example

During a lab, a student collected a gas over water. The room temperature was 21°C and room pressure was 102.90 kPa. After equalizing the water levels, the volume of gas collected was 325 mL. What volume would the dry gas occupy at STP?

Review the following video to investigate some principles regarding pressure - including John Dalton's Law of Partial Pressures, vapour pressure - and demonstrating the method for collecting gas over water.

This lesson is now complete. Consolidate your learning by evaluating the self-check below and the assigned practice questions.

Self-Check

(This is a self-assessment of your learning – you do not need to submit this).

How prepared am I to start my homework? Can I ...

- use Dalton’s Law to find the partial pressure of a gas?
- use Dalton’s Law of Partial pressures to find the pressure of a dry gas when collected over water?

Practice

Using your textbook, complete the following questions:

- WS “Dalton’s Law of Partial Pressures”

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