Chemistry · 9th Grade · Quantifying Chemistry: Stoichiometry · Weeks 10-18

Dalton's Law of Partial Pressures

Students will calculate the total pressure of a gas mixture and the partial pressure of individual gases.

• Common Core State Standards
• HS-PS1-3
• STD.CCSS.MATH.CONTENT.HSN.Q.A.3

About This Topic

Dalton's Law of Partial Pressures states that the total pressure of a gas mixture equals the sum of the pressures each gas would exert if it occupied the container alone. This additive relationship emerges because gas particles in an ideal mixture do not interact with each other; each gas behaves independently. In the US 9th grade chemistry curriculum, this principle appears in stoichiometry problems and in laboratory contexts where gases are collected over water.

The most common application students encounter is pneumatic trough collection: when a gas is bubbled into an inverted flask over water, water vapor mixes with the collected gas. To find the pressure of the pure collected gas, students subtract the water vapor pressure (which depends on temperature and is found in reference tables) from the measured total pressure.

At the molecular level, Dalton's Law reflects the fact that gas molecules in a mixture each contribute to total pressure through their own collisions with container walls. Active learning approaches that require students to reason from the molecular level to the macroscopic calculation build the conceptual foundation that supports correct problem setup.

Key Questions

1. Explain Dalton's Law of Partial Pressures at the molecular level.
2. Calculate the total pressure of a gas mixture given the partial pressures of its components.
3. Analyze how gas collection over water is affected by water vapor pressure.

Learning Objectives

• Explain Dalton's Law of Partial Pressures by describing the independent behavior of gas molecules in a mixture at the molecular level.
• Calculate the total pressure of a gas mixture given the partial pressures of its individual components using the additive relationship.
• Analyze gas collection over water experiments by accounting for the partial pressure of water vapor at a given temperature.
• Determine the partial pressure of a specific gas in a mixture when provided with the total pressure and the mole fractions of all components.

Before You Start

Ideal Gas Law (PV=nRT)

Why: Students need to understand the relationship between pressure, volume, temperature, and moles of a gas to apply Dalton's Law.

Introduction to Gas Properties

Why: Students must have a foundational understanding of what pressure is at the molecular level (collisions of particles) to grasp partial pressures.

Key Vocabulary

Dalton's Law of Partial Pressures	The total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of each individual gas in the mixture.
Partial Pressure	The pressure that a single gas in a mixture would exert if it were the only gas present in the container.
Mole Fraction	The ratio of the number of moles of one component in a mixture to the total number of moles of all components in the mixture; it is unitless.
Water Vapor Pressure	The pressure exerted by water molecules in the gaseous phase above a liquid or solid surface; it is dependent on temperature.

Watch Out for These Misconceptions

Common Misconception

The total pressure of a gas mixture is the average of the partial pressures, not the sum.

What to Teach Instead

Dalton's Law is additive: P_total = P1 + P2 + P3 + ... Each gas contributes its full partial pressure to the total. The confusion with averaging likely comes from temperature mixing rules. Whiteboard problems where students add three or four components to reach a total pressure make the additive rule concrete.

Common Misconception

Water vapor pressure stays the same regardless of temperature when collecting gas over water.

What to Teach Instead

Water vapor pressure increases with temperature and must be looked up for the specific temperature of the experiment. This is why reference tables are provided. Lab exercises where students measure at two temperatures and compare corrections build awareness of this variable.

Active Learning Ideas

Whiteboard Problem: Gas Collection Over Water

Provide groups with a scenario: a gas collected over water at a measured barometric pressure and temperature. Each group works on a mini-whiteboard to subtract water vapor pressure (from a provided table) to find the pressure of the dry gas. Groups display boards simultaneously for class comparison and discussion of any discrepancies.

20 min·Small Groups

Think-Pair-Share: Molecular-Level Reasoning

Project a particle diagram of a gas mixture containing two gases (shown in two colors). Ask students individually to predict which gas contributes more to total pressure and why. Pairs discuss, then the class connects particle count per volume to partial pressure, reinforcing the mole fraction relationship.

15 min·Pairs

Gallery Walk: Partial Pressure Scenarios

Post five different gas mixture problems around the room (varying number of components, known/unknown variables). Students rotate in pairs, solving one problem at each station and annotating the solutions left by the previous pair. The final rotation is a whole-class review of the most contested station.

25 min·Pairs

Real-World Connections

• Scuba divers must understand partial pressures to manage the gases they breathe at different depths, preventing conditions like nitrogen narcosis or oxygen toxicity.
• Atmospheric chemists analyze the composition of air, calculating the partial pressures of gases like oxygen, nitrogen, and carbon dioxide to model air quality and predict weather patterns.

Assessment Ideas

Quick Check

Present students with a scenario: A 5.0 L container holds 2.0 moles of Helium and 3.0 moles of Neon at 25°C. Ask them to calculate the partial pressure of Helium and the total pressure of the mixture, assuming ideal gas behavior. Review calculations as a class.

Exit Ticket

Provide students with a reference table for water vapor pressure at various temperatures. Give them a problem: A gas is collected over water at 20°C, and the total pressure inside the collection vessel is 750 mmHg. Ask them to calculate the partial pressure of the dry gas collected.

Discussion Prompt

Pose the question: 'Why is it important to consider water vapor pressure when collecting gases over water in a laboratory setting?' Facilitate a discussion where students explain the concept at both the molecular and macroscopic calculation levels.

Frequently Asked Questions

Why do we subtract water vapor pressure when collecting gas over water?

When gas is collected over water, some water molecules evaporate and mix with the collected gas. The measured pressure in the container includes both the collected gas and water vapor. Subtracting the water vapor pressure (found in a table for the given temperature) gives the pressure of the dry collected gas alone, which is what's needed for calculations.

How do you calculate partial pressure from mole fractions?

The partial pressure of a gas equals its mole fraction (moles of that gas divided by total moles) multiplied by total pressure: P_i = X_i x P_total. For example, if nitrogen makes up 78% of air by mole at 1 atm, its partial pressure is 0.78 atm. This relationship connects Dalton's Law to the composition of gas mixtures.

Does Dalton's Law apply to real gases or only ideal gases?

Dalton's Law is strictly accurate only for ideal gases, where particles do not interact. For most real gases at low to moderate pressures, it is an excellent approximation. Deviations occur when gases interact (e.g., polar gas molecules) or under high pressure, where intermolecular forces between different gas molecules affect the measured partial pressures.

How does active learning improve understanding of Dalton's Law?

Dalton's Law involves two distinct skills: molecular-level reasoning (why pressures add) and algebraic manipulation (solving for unknowns). Students who only see worked examples often apply the formula without understanding the molecular picture. Think-Pair-Share discussions on particle diagrams build the conceptual foundation before students attempt calculations.