Ideal Gas Law MCAT: PV = nRT and Boyle, Charles, Avogadro

Ideal Gas Law and the Simple Gas Laws for the MCAT

By StudyStore · Published 2026-11-04 · 2 min read · 403 words

The ideal gas law is PV = nRT, where P is pressure, V is volume, n is moles, T is absolute temperature in kelvin, and R is the gas constant. Every simple gas law is this equation with some variables held constant.

The simple gas laws

Law	Held constant	Relationship
Boyle's	n, T	P and V are inversely proportional: P1V1 = P2V2
Charles's	n, P	V and T are directly proportional: V1/T1 = V2/T2
Gay-Lussac's	n, V	P and T are directly proportional: P1/T1 = P2/T2
Avogadro's	P, T	V and n are directly proportional: V1/n1 = V2/n2
Combined	n	P1V1/T1 = P2V2/T2

Always convert temperature to kelvin (K = degrees C + 273). Using Celsius is the most common error.

Useful values

R = 0.0821 L atm / (mol K) or 8.314 J / (mol K).
At standard temperature and pressure (0 degrees C, 1 atm), one mole of an ideal gas occupies about 22.4 L.
1 atm = 760 mmHg = 760 torr, about 101.3 kPa.

Dalton's law of partial pressures

In a mixture, each gas contributes as if it were alone, and the total pressure is the sum of the partial pressures. The partial pressure of gas A is its mole fraction times the total pressure: P_A = X_A x P_total. This is how alveolar oxygen and nitrogen pressures are worked out in respiratory questions.

Ideal versus real gases

The ideal model assumes gas particles have negligible volume and no intermolecular attractions, and collide elastically. Real gases deviate at high pressure (particle volume matters) and low temperature (attractions matter). A gas behaves most ideally at high temperature and low pressure.

Kinetic molecular theory

Average kinetic energy is proportional to absolute temperature and does not depend on the identity of the gas. At the same temperature, lighter molecules move faster. Graham's law says the rate of effusion is inversely proportional to the square root of molar mass.

Common traps

Using Celsius. Convert to kelvin first.
Forgetting that doubling the temperature in kelvin, not Celsius, doubles the volume.
Thinking heavier gases have more kinetic energy at the same temperature. They are slower, not more energetic.

One-minute recap

PV = nRT; hold variables constant to get each law.
Boyle inverse P-V; Charles and Gay-Lussac direct with T in kelvin.
Total pressure is the sum of partial pressures.
Real gases deviate at high pressure and low temperature.

Check yourself: 5-question mini-quiz

Try each question first, then click it to reveal the answer.

1. What is the relationship between pressure and volume at constant temperature and moles?

They are inversely proportional (Boyle's law).

2. What unit must temperature be in for gas law calculations?

Kelvin.

3. What is the volume of one mole of ideal gas at STP?

About 22.4 L.

4. How is the partial pressure of a gas in a mixture found?

Mole fraction times total pressure (Dalton's law).

5. Under what conditions do real gases deviate most from ideal behavior?

High pressure and low temperature.

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Written by StudyStore from standard scientific facts. Study aid only. Always check values against the reference your exam provides.