The gas laws look like four separate formulas to memorize, but three of them are really just special cases of the fourth. Once that relationship is clear, deciding which one to use stops being a guessing game.
Boyle's Law: pressure and volume
At constant temperature and amount of gas, pressure and volume are inversely related: P1V1 = P2V2. Squeeze a gas into a smaller volume and its pressure goes up.
Charles's Law: volume and temperature
At constant pressure and amount of gas, volume and temperature are directly related: V1/T1 = V2/T2. Heat a gas at constant pressure and it expands.
The Combined Gas Law
When both temperature and pressure change and the amount of gas stays fixed, the combined gas law handles it: P1V1/T1 = P2V2/T2. It's exactly Boyle's and Charles's Laws merged into one relationship.
The Ideal Gas Law: when the amount of gas isn't fixed
PV = nRT is the most general version, used whenever moles of gas (n) are part of the problem, or when a single set of conditions needs to be solved rather than compared before and after. R is the gas constant, most commonly 0.0821 L·atm/(mol·K) when pressure is in atmospheres.
Worked example
Problem
Setup
Solve
Where students actually lose points here
Forgetting to convert temperature to Kelvin
Every gas law calculation needs temperature in Kelvin, never Celsius. This is the single most common gas law mistake, especially under time pressure.
Picking the wrong law for what's actually changing
Boyle's Law needs constant temperature, Charles's Law needs constant pressure. Using either one when that condition isn't actually held constant gives a wrong answer even with correct arithmetic.
Mismatching units with the gas constant
The value of R depends on which units are being used for pressure and volume. Using atmospheres with a version of R meant for different units throws the whole calculation off.
Gas laws come back in unexpected places
Gas behavior reappears in kinetic molecular theory, effusion problems, and even some equilibrium calculations later in the course.
Common questions
When do you use the ideal gas law instead of the combined gas law?
Use the ideal gas law, PV = nRT, when moles of gas are involved or you only have one set of conditions to work with. Use the combined gas law when comparing two sets of conditions (before and after a change) for a fixed amount of gas.
Why must temperature be in Kelvin for gas law calculations?
The gas laws are based on relationships that only hold true on an absolute temperature scale, where zero actually means zero energy. Celsius has a different zero point, so using it directly gives incorrect ratios.
What is STP?
Standard temperature and pressure, defined as 0°C (273 K) and 1 atmosphere. It's a common reference point for comparing gas volumes and is often used as a starting condition in gas law problems.