Question
A gas occupies 2 L at 300 K. If pressure is kept constant, what volume will it occupy at 600 K?
Solution — Step by Step
Pressure is constant throughout — this is the key condition that tells us Charles’ Law applies. We have initial volume , initial temperature , and final temperature .
At constant pressure, volume is directly proportional to absolute temperature:
The ratio stays constant. This is Charles’ Law.
Multiply both sides by :
Final Answer: V₂ = 4 L
Why This Works
Charles’ Law captures a simple physical reality: when you heat a gas at constant pressure, the gas molecules move faster and need more space to maintain the same pressure against the container walls. Volume and temperature rise together in a direct proportion.
The catch — and this trips up a lot of students — is that this proportionality only holds with Kelvin temperatures, not Celsius. Kelvin is the absolute scale, so 0 K is true zero molecular motion. Celsius doesn’t have that property.
Here, the temperature doubled (300 K → 600 K), so the volume doubled (2 L → 4 L). Clean ratio, clean answer. This type of calculation is a staple of CBSE Class 11 Chapter 5 numericals.
Alternative Method
We can use the proportionality directly without the full formula. Since at constant pressure:
So .
When is a simple multiple of , just use the ratio directly. No need to plug into the full formula. Here, temperature doubled → volume doubled. These “clean ratio” questions are very common in board exams and JEE Main.
Common Mistake
Using Celsius instead of Kelvin. If a student accidentally writes and (which is how the question sometimes appears), the ratio gives a nonsense answer. Always convert to Kelvin first: . Charles’ Law breaks with Celsius — the direct proportionality only holds on the absolute scale.