What is Dalton's Law of Partial Pressures?

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Grade Level:

Class 12

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Definition

What is it?

Dalton's Law of Partial Pressures states that in a mixture of non-reacting gases, the total pressure exerted by the mixture is simply the sum of the individual pressures that each gas would exert if it were alone in the container. Think of it like each gas doing its own job to create pressure, and the total pressure is the sum of all their efforts.

Simple Example

Quick Example

Imagine you have a small room (your gas container) and two friends, Rahul and Priya, are inside. Rahul is pushing against one wall with a certain force, and Priya is pushing against another wall with her own force. If you want to know the total force pushing against all the walls, you simply add Rahul's force and Priya's force. Each friend contributes independently to the total force, just like each gas contributes independently to the total pressure.

Worked Example

Step-by-Step

Let's say a gas cylinder contains a mixture of Oxygen (O2) and Nitrogen (N2). The pressure exerted by Oxygen alone is 0.5 atmospheres (atm), and the pressure exerted by Nitrogen alone is 0.7 atm.

Step 1: Identify the partial pressures of each gas.
Partial pressure of Oxygen (PO2) = 0.5 atm
Partial pressure of Nitrogen (PN2) = 0.7 atm
---Step 2: Apply Dalton's Law, which states that Total Pressure (P_total) = PO2 + PN2.
---Step 3: Add the partial pressures together.
P_total = 0.5 atm + 0.7 atm
---Step 4: Calculate the sum.
P_total = 1.2 atm

So, the total pressure inside the cylinder is 1.2 atm.

Why It Matters

Understanding Dalton's Law is crucial for scientists and engineers. For example, in Medicine, doctors use it to understand how gases like oxygen and carbon dioxide behave in our lungs. In Space Technology, it helps design life support systems for astronauts, ensuring they have the right mix of gases to breathe in a spacecraft.

Common Mistakes

MISTAKE: Students often think that gases react with each other and change their individual pressures when mixed. | CORRECTION: Dalton's Law applies ONLY to non-reacting gases. Each gas acts independently, and its partial pressure doesn't change just because other gases are present.

MISTAKE: Assuming the partial pressure of a gas is the same as its concentration or percentage in the mixture. | CORRECTION: While related, partial pressure is a measure of the pressure exerted by that specific gas, not just its amount. You need to use the ideal gas law (PV=nRT) for each gas to find its partial pressure if you only have concentration.

MISTAKE: Forgetting to use consistent units for pressure (e.g., mixing atmospheres with Pascals). | CORRECTION: Always ensure all pressures are in the same unit (like atm, kPa, or mmHg) before adding them to find the total pressure.

Practice Questions

Try It Yourself

QUESTION: A balloon contains a mixture of Helium and Argon. The partial pressure of Helium is 0.8 atm and the partial pressure of Argon is 0.3 atm. What is the total pressure in the balloon? | ANSWER: 1.1 atm

QUESTION: A diving tank has a gas mixture with Oxygen at 1.5 atm, Nitrogen at 3.5 atm, and a small amount of Carbon Dioxide at 0.05 atm. What is the total pressure inside the tank? | ANSWER: 5.05 atm

QUESTION: In a container, the total pressure of a gas mixture is 2.2 atm. If the mixture contains two gases, A and B, and the partial pressure of gas A is 1.4 atm, what is the partial pressure of gas B? | ANSWER: 0.8 atm

MCQ

Quick Quiz

Which of the following statements is true about Dalton's Law of Partial Pressures?

It applies to gases that react with each other.

The total pressure is the product of individual partial pressures.

The total pressure is the sum of the partial pressures of non-reacting gases.

It states that all gases in a mixture have the same partial pressure.

The Correct Answer Is:

Dalton's Law specifically applies to non-reacting gases, and it states that their individual partial pressures add up to the total pressure. Options A, B, and D describe incorrect aspects of the law.

Real World Connection

In the Real World

When you get an oxygen cylinder in a hospital in India, it's not always pure oxygen; sometimes it's a mixture of oxygen and air (which is mostly nitrogen). Doctors need to know the 'partial pressure' of oxygen to ensure the patient is getting enough. Similarly, scuba divers breathing a special gas mix called 'Nitrox' rely on Dalton's Law to understand the partial pressures of oxygen and nitrogen at different depths to stay safe.

Key Vocabulary

Key Terms

PARTIAL PRESSURE: The pressure exerted by an individual gas in a mixture of gases. | TOTAL PRESSURE: The sum of all the individual partial pressures in a gas mixture. | NON-REACTING GASES: Gases that do not chemically combine with each other when mixed. | ATMOSPHERE (atm): A common unit for measuring pressure, approximately equal to the pressure at sea level.

What's Next

What to Learn Next

Great job understanding Dalton's Law! Next, you should explore the Ideal Gas Law (PV=nRT). It will help you calculate partial pressures when you know the amount of each gas, and connect pressure, volume, temperature, and moles of a gas in a powerful way.