Combined Gas Law
Combines Boyle, Charles, and Gay-Lussac laws.
Formula
Variables
Example
General gas state change
How it works
The combined gas law links pressure, volume, and absolute temperature for a fixed amount of gas, so you can predict how changing two of these quantities affects the third. It comes from combining Boyle’s law, Charles’s law, and Gay-Lussac’s law, each of which describes one relationship when the other variables are held constant. The temperatures must be in Kelvin because gas-law ratios only work with an absolute temperature scale.
When to use it
Use this when a gas sample stays the same amount but its pressure, volume, and temperature all change between two states. It is common in chemistry problems about sealed containers, weather balloons, syringes, or compressed gases.
Worked example, step by step
A gas has an initial pressure of 1.20 atm, volume of 3.50 L, and temperature of 290 K. It is changed to a volume of 2.10 L and a temperature of 350 K. Find the new pressure. Use the combined gas law: P1V1/T1 = P2V2/T2 Substitute the known values: (1.20 atm)(3.50 L)/(290 K) = (P2)(2.10 L)/(350 K) Solve for P2: P2 = [(1.20)(3.50)(350)] / [(290)(2.10)] P2 = 1470 / 609 P2 = 2.41 atm Final answer: The new pressure is 2.41 atm.
Common mistakes to avoid
- Using Celsius instead of Kelvin for temperature
- Forgetting to keep the amount of gas constant when applying the formula
- Mixing up which values are initial (1) and final (2) in the calculation
Did You Know?
The combined gas law merges Boyle's, Charles's, and Gay-Lussac's laws into one tidy relationship.
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