Table of Contents
What Is the Ideal Gas Law?
The ideal gas law is a fundamental equation in thermodynamics and chemistry that describes the behavior of an ideal gas. It combines Boyle's Law, Charles's Law, and Avogadro's Law into a single equation relating pressure, volume, amount of substance, and temperature.
An ideal gas is a theoretical gas whose molecules occupy negligible space and have no intermolecular forces. While no real gas behaves perfectly ideally, most gases at moderate temperatures and low pressures approximate ideal behavior closely enough for practical calculations.
The Formula
Where P is pressure, V is volume, n is the number of moles, R is the universal gas constant, and T is the absolute temperature in Kelvin.
Gas Constant Values
| R Value | Units |
|---|---|
| 8.314 | J/(mol·K) = Pa·m³/(mol·K) |
| 0.08206 | L·atm/(mol·K) |
| 62.36 | L·mmHg/(mol·K) |
| 1.987 | cal/(mol·K) |
Special Cases of the Ideal Gas Law
- Boyle's Law: At constant T and n: P&sub1;V&sub1; = P&sub2;V&sub2;
- Charles's Law: At constant P and n: V&sub1;/T&sub1; = V&sub2;/T&sub2;
- Gay-Lussac's Law: At constant V and n: P&sub1;/T&sub1; = P&sub2;/T&sub2;
- Avogadro's Law: At constant T and P: V&sub1;/n&sub1; = V&sub2;/n&sub2;
- Combined Gas Law: At constant n: P&sub1;V&sub1;/T&sub1; = P&sub2;V&sub2;/T&sub2;
Frequently Asked Questions
What is the volume of one mole of gas at STP?
At standard temperature and pressure (0 degrees C = 273.15 K, 1 atm), one mole of an ideal gas occupies exactly 22.414 liters. This is known as the molar volume at STP and is the same for all ideal gases regardless of their chemical identity.
Why must temperature be in Kelvin?
The ideal gas law requires an absolute temperature scale because gas properties are directly proportional to absolute temperature. At 0 K (absolute zero), an ideal gas would have zero volume and zero pressure. Using Celsius or Fahrenheit would give meaningless results because they have arbitrary zero points.
How accurate is the ideal gas law?
The ideal gas law is accurate to within a few percent for most gases at pressures below about 5 atm and temperatures well above their boiling points. For more precise work or extreme conditions, use the van der Waals equation, Redlich-Kwong, or Peng-Robinson equations of state.