Ideal Gas Law Calculator – Solve PV = nRT for Any Variable
Apply the ideal gas law PV = nRT to find any unknown gas property. Our calculator solves for pressure, volume, temperature, or amount of gas in chemistry and physics.
How to Use This Ideal Gas Law Calculator
Choose which variable to solve for
Select pressure (P), volume (V), amount of substance (n), or temperature (T) from the dropdown menu.
Enter the three known values
Fill in the remaining fields. Temperature is entered in Celsius and converted to Kelvin automatically.
Get your calculated result
The calculator applies PV = nRT to find the unknown variable with proper SI units.
Common Gas Properties Reference
| Gas | Molar Mass (g/mol) | Boiling Point (°C) | Density at STP (g/L) |
|---|---|---|---|
| Helium (He) | 4.00 | -268.9 | 0.179 |
| Nitrogen (N₂) | 28.01 | -195.8 | 1.25 |
| Oxygen (O₂) | 32.00 | -183.0 | 1.43 |
| Carbon Dioxide (CO₂) | 44.01 | -78.5 (sublimes) | 1.98 |
| Hydrogen (H₂) | 2.02 | -252.9 | 0.090 |
| Methane (CH₄) | 16.04 | -161.5 | 0.717 |
| Air (average) | 28.97 | -194.3 | 1.29 |
STP = Standard Temperature and Pressure (0°C, 1 atm or 101.325 kPa)
Understanding the Ideal Gas Law
What Is the Ideal Gas Law?
The ideal gas law relates pressure, volume, temperature, and amount of gas in one equation: PV = nRT. It combines Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law. Real gases behave like ideal gases at high temperatures and low pressures.
The Variables Explained
P is pressure in pascals (Pa). V is volume in cubic meters (m³). n is the amount of substance in moles (mol). T is absolute temperature in kelvin (K). R is the universal gas constant, 8.314 J/(mol·K).
Temperature Must Be in Kelvin
The ideal gas law requires absolute temperature. Convert Celsius to Kelvin by adding 273.15. Zero kelvin (absolute zero) is -273.15°C, the point where molecular motion theoretically stops. This calculator handles the conversion for you.
When Real Gases Deviate
Ideal gas behavior breaks down at high pressures or low temperatures. Gas molecules have volume and attract each other. The van der Waals equation corrects for these effects. For most classroom problems and moderate conditions, the ideal gas law works well.
Tips for Gas Law Calculations
Watch your units
Always convert to SI units before calculating: pascals for pressure, cubic meters for volume, kelvin for temperature. This calculator uses SI units internally.
Know your gas constant
R = 8.314 J/(mol·K) when using pascals and cubic meters. If using atmospheres and liters, R = 0.0821 L·atm/(mol·K). Match R to your units.
Use molar mass to find moles
If you have mass instead of moles, divide by molar mass: n = mass / molar mass. For example, 32 g of O₂ equals 1 mole (32 g ÷ 32 g/mol).
Check if your answer makes sense
Does the pressure seem reasonable? Is the volume plausible? If you get negative kelvin or impossible values, recheck your inputs and unit conversions.
Frequently Asked Questions
What is the ideal gas law used for?
The ideal gas law predicts how gases behave when conditions change. Chemists use it to calculate reactant volumes in gas reactions. Engineers apply it to design pneumatic systems. Meteorologists use it to model atmospheric behavior.
What is the value of R in the ideal gas law?
R equals 8.314 J/(mol·K) in SI units. This is the most common value for physics and chemistry calculations. In other unit systems: 0.0821 L·atm/(mol·K), 62.36 L·torr/(mol·K), or 1.987 cal/(mol·K).
When does the ideal gas law not work?
The ideal gas law fails at very high pressures (molecules are crowded) or very low temperatures (molecules attract strongly). Near the condensation point, real gases deviate significantly. The van der Waals equation handles these cases better.
How do I convert between pressure units?
1 atm = 101,325 Pa = 101.325 kPa = 760 mmHg = 760 torr = 14.7 psi. To convert, multiply by the appropriate factor. For example, 2 atm × 101,325 = 202,650 Pa.
What is molar volume at STP?
At standard temperature and pressure (0°C, 1 atm), one mole of any ideal gas occupies 22.4 liters. This is useful for quick estimates. At SATP (25°C, 1 bar), molar volume is about 24.8 L/mol.
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