# Ideal gas law

The ideal gas law is PV = nRT. P is pressure, V is volume, n is moles, T is temperature in kelvin, and R is the gas constant, 0.08206 L·atm/(mol·K). It works well at low pressure and high temperature.

Chemistry (university) · Part 03: Intermolecular forces, gases and solutions · Card 4 of 9 · Last updated 2026-10-19

## Analogy

A balloon: heat it (T up) and it grows; squeeze it (P up) and it shrinks.

## Example

**Molar volume at 0 °C**

n = 1.00 mol, T = 273.15 K, P = 1.00 atm
V = nRT/P
V = (1.00 × 0.08206 × 273.15) / 1.00
V = 22.4 L

## In practice

Airbag designers use the law to work out how much gas a chemical reaction must release to fill the bag.

## Practice

### Flashcards

- **State the ideal gas law.** PV = nRT, with T in kelvin.

### Interview questions

**Calculate the pressure of 0.500 mol of an ideal gas in 10.0 L at 300 K.**

- Use PV = nRT
- R = 0.08206 L·atm/(mol·K)
- P = 0.500 × 0.08206 × 300 / 10.0
- P ≈ 1.23 atm

## Questions people ask

### Why must temperature be in kelvin?

Gas volume and pressure are proportional to absolute temperature. The kelvin scale starts at absolute zero, where gas particles would have no motion. Celsius has a zero that is arbitrary, so using it would give wrong answers or even negative volumes. Add 273.15 to a Celsius temperature.

### What is STP?

STP stands for standard temperature and pressure. A common definition is 0 °C (273.15 K) and 1 atm, where one mole of an ideal gas takes up 22.4 L. IUPAC now uses 1 bar instead of 1 atm, giving 22.7 L, so always check which definition a problem uses.

## Related concepts

- [Dalton's law of partial pressures](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-5)
- [Kinetic molecular theory](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-6)
- [Real gases and van der Waals equation](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-7)

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