What is the Combined Gas Law? | Simple Explanation for Class 8

S4-SA2-0474

What is the Combined Gas Law (chemistry)?

Grade Level:

Class 8

Space Technology, EVs, Climate Change, Biotechnology, HealthTech, Robotics, Chemistry, Physics

Definition

What is it?

The Combined Gas Law helps us understand how the pressure, volume, and temperature of a fixed amount of gas are related when they all change at the same time. It combines Boyle's Law, Charles's Law, and Gay-Lussac's Law into one simple equation. This law is useful for situations where a gas is moved from one condition to another.

Simple Example

Quick Example

Imagine you have a small air pump for your bicycle tyre. When you push the handle, the volume of air inside the pump decreases, its pressure increases, and it might even feel a little warm due to the temperature change. The Combined Gas Law helps explain how these three factors – pressure, volume, and temperature – change together for the air inside the pump.

Worked Example

Step-by-Step

A gas occupies a volume of 2.0 litres at 300 K temperature and 1.5 atm pressure. If the gas is compressed to 1.0 litre and its temperature is raised to 400 K, what will be the new pressure?

Step 1: Write down the initial conditions (P1, V1, T1) and final conditions (P2, V2, T2).
P1 = 1.5 atm, V1 = 2.0 L, T1 = 300 K
V2 = 1.0 L, T2 = 400 K, P2 = ?

---Step 2: Recall the Combined Gas Law formula: (P1 * V1) / T1 = (P2 * V2) / T2.

---Step 3: Substitute the known values into the formula.
(1.5 atm * 2.0 L) / 300 K = (P2 * 1.0 L) / 400 K

---Step 4: Simplify the left side of the equation.
3.0 / 300 = (P2 * 1.0) / 400
0.01 = (P2 * 1.0) / 400

---Step 5: Isolate P2 to find its value.
P2 = 0.01 * 400 / 1.0
P2 = 4.0 atm

---Answer: The new pressure will be 4.0 atm.

Why It Matters

Understanding the Combined Gas Law is crucial for engineers designing space vehicles or scuba diving tanks, where gases operate under extreme conditions. It's also important for scientists working on climate change models, understanding how atmospheric gases behave. Careers in aerospace, environmental science, and even medical technology (like oxygen masks) rely on this knowledge.

Common Mistakes

MISTAKE: Using Celsius for temperature in the calculations. | CORRECTION: Always convert temperature to Kelvin (K) before using it in the Combined Gas Law formula. Remember, K = degrees Celsius + 273.15.

MISTAKE: Mixing up units for pressure or volume (e.g., using atm for P1 and kPa for P2). | CORRECTION: Ensure all pressure units are the same (e.g., both in atm or both in Pa) and all volume units are the same (e.g., both in litres or both in mL) before starting the calculation.

MISTAKE: Incorrectly rearranging the formula to solve for an unknown variable. | CORRECTION: Practice isolating the unknown variable step-by-step. For example, if solving for P2, multiply both sides by T2 and divide by V2.

Practice Questions

Try It Yourself

QUESTION: A balloon contains 10 L of air at 27 degrees Celsius and 1 atm pressure. If the balloon is taken to a place where the temperature is 77 degrees Celsius and the pressure is 2 atm, what will be its new volume? | ANSWER: 6.25 L

QUESTION: A gas occupies 500 mL at 20 degrees Celsius and 750 mmHg. What temperature (in Kelvin) is needed to change its volume to 400 mL at 780 mmHg? | ANSWER: 247.9 K

QUESTION: An unknown gas has an initial volume of 3.5 L at 25 degrees Celsius and 100 kPa. If the temperature is increased by 50 degrees Celsius and the pressure is reduced to 80 kPa, what is the final volume of the gas? | ANSWER: 5.47 L

MCQ

Quick Quiz

Which of the following conditions MUST be constant for the Combined Gas Law to be applied?

Temperature only

Pressure and Volume only

The amount of gas (moles)

All of the above

The Correct Answer Is:

The Combined Gas Law describes how pressure, volume, and temperature change for a FIXED amount of gas. If the amount of gas changes, the law does not apply directly.

Real World Connection

In the Real World

When you use a pressure cooker at home, the steam inside is a gas. As the temperature rises, the pressure also increases, and the volume is fixed. While a pressure cooker is often explained by Gay-Lussac's Law, the Combined Gas Law helps us understand what would happen if the cooker's volume could also change, for instance, in industrial-scale cooking or chemical processing where conditions are carefully controlled.

Key Vocabulary

Key Terms

PRESSURE: The force exerted by gas particles on the walls of their container. | VOLUME: The space occupied by the gas. | TEMPERATURE: A measure of the average kinetic energy of gas particles, always in Kelvin for gas laws. | KELVIN: The absolute temperature scale where 0 K is absolute zero.

What's Next

What to Learn Next

Great job understanding the Combined Gas Law! Next, you can explore the Ideal Gas Law, which is an even more comprehensive law that also considers the amount of gas (in moles). This will help you solve even more complex problems in chemistry and physics!