Chemistry overview
What is the ideal gas law?
The ideal gas law relates the pressure, volume, amount, and absolute temperature of a gas. It combines several simpler gas laws into one equation that can solve many introductory chemistry and laboratory problems.
The equation models gas particles as having negligible volume and no intermolecular attraction. It provides a useful approximation when a real gas behaves close to those assumptions.
Formula
Ideal gas law formula
Ideal gas lawP × V = n × R × T
- P is absolute gas pressure.
- V is the volume occupied by the gas.
- n is the amount of gas in moles.
- R is the ideal gas constant.
- T is absolute temperature in kelvin.
This calculator uses R = 8.314462618 kPa·L/(mol·K). It converts entered values to compatible internal units before solving the equation.
Rearranged equations
Solve for any ideal gas variable
Pressure
P = nRT ÷ V
Divide moles times the gas constant and temperature by volume.
Volume
V = nRT ÷ P
Divide moles times the gas constant and temperature by pressure.
Moles
n = PV ÷ RT
Divide pressure times volume by the gas constant and temperature.
Temperature
T = PV ÷ nR
Divide pressure times volume by moles and the gas constant.
Supported units
Pressure, volume, and temperature units
- Pressure: pascals, kilopascals, bar, atmospheres, or millimeters of mercury.
- Volume: milliliters, liters, or cubic meters.
- Amount of gas: moles.
- Temperature: kelvin, Celsius, or Fahrenheit.
Temperature is always converted to kelvin for the calculation because gas-law equations require an absolute temperature scale.
Worked example
Calculate gas pressure at standard temperature
Find the pressure of 1 mole of an ideal gas occupying 22.414 liters at 273.15 kelvin.
- Rearrange the equation for pressure: P = nRT ÷ V.
- Substitute the values: P = 1 × 8.314462618 × 273.15 ÷ 22.414.
- Complete the calculation: P ≈ 101.325 kPa.
- This pressure is approximately 1 atmosphere.
Chemistry guidance
Common ideal gas law mistakes
- Using gauge pressure instead of absolute pressure.
- Substituting Celsius or Fahrenheit directly without converting to kelvin.
- Mixing pressure and volume units that do not match the selected gas constant.
- Entering particle count instead of amount in moles.
- Rounding converted values too early in the calculation.
Model assumptions
Ideal gas law assumptions and limitations
- Gas particles are treated as having negligible individual volume.
- Intermolecular attractions and repulsions are ignored.
- Particle collisions are assumed to be perfectly elastic.
- The gas is assumed to be in a uniform equilibrium state.
- Accuracy may decrease at high pressure, low temperature, or near a phase change.
- All entered quantities must describe the same gas sample and physical state.
Questions and answers
Ideal gas law calculator FAQ
What formula does the ideal gas law calculator use?
The calculator uses PV = nRT, where P is absolute pressure, V is gas volume, n is the amount of gas in moles, R is the ideal gas constant, and T is absolute temperature in kelvin.
Can the ideal gas law calculator solve for any variable?
Yes. Enter three known values and select pressure, volume, moles, or temperature as the unknown variable.
Does temperature need to be entered in kelvin?
No. You can enter temperature in kelvin, Celsius, or Fahrenheit. The calculator converts the value to kelvin internally before applying the formula.
When is the ideal gas law less accurate?
The ideal gas law becomes less accurate for gases at high pressure, very low temperature, or near condensation because real molecular volume and intermolecular forces become significant.