Master the Ideal Gas Law
Everything you need to know about calculating pressure, volume, temperature, and moles. Navigate the PV = nRT formula and unit conversions with confidence.
Your Step-by-Step Calculation Roadmap
Solving gas law problems requires precision and consistent units. Follow this structured roadmap to ensure accurate, error-free calculations every time.
The Calculation Process
Five essential phases to solving PV = nRT
Identify the Known Variables
Determine which variables you have: Pressure (P), Volume (V), number of moles (n), or Temperature (T). Identify the single unknown variable you need to solve for.
Convert Temperature to Kelvin
Always convert Celsius or Fahrenheit to Kelvin before calculating. For Celsius, add 273.15 to the temperature value. The Ideal Gas Law requires absolute temperature.
Select the Correct Gas Constant (R)
Choose the appropriate value for the ideal gas constant (R) that matches your pressure and volume units (e.g., 0.08206 L·atm/mol·K or 8.314 J/mol·K).
Rearrange the Formula
Algebraically rearrange PV = nRT to isolate your unknown variable. For example, to find Volume, use V = (nRT) / P. To find Pressure, use P = (nRT) / V.
Calculate and Verify Units
Plug in the values, perform the calculation, and ensure the final answer has the correct units and appropriate significant figures based on your input data.
Pre-Calculation Checklist
Ensure these boxes are ticked before solving
Common Gas Constant (R) Values
A summary of the primary values for the ideal gas constant based on different unit combinations.
| Value of R | Units | When to Use |
|---|---|---|
| 0.08206 | L·atm / (mol·K) | When pressure is in atmospheres (atm) and volume in litres (L). |
| 8.314 | J / (mol·K) or L·kPa / (mol·K) | When using SI units (Joules) or pressure in kilopascals (kPa). |
| 62.36 | L·mmHg / (mol·K) or L·Torr / (mol·K) | When pressure is measured in millimetres of mercury (mmHg) or Torr. |
| 0.08314 | L·bar / (mol·K) | When pressure is measured in bars and volume in litres (L). |
| 1.987 | cal / (mol·K) | When energy calculations are required in calories rather than Joules. |
Ideal Gas Law FAQ
Answers to the most frequently asked questions about the Ideal Gas Law and gas calculations.
The Ideal Gas Law formula is PV = nRT, where P is pressure, V is volume, n is the number of moles of gas, R is the ideal gas constant, and T is the absolute temperature in Kelvin.
The value of R depends on the units used. The most common values are 0.08206 L·atm/(mol·K) when using atmospheres and litres, and 8.314 J/(mol·K) or L·kPa/(mol·K) when using SI units.
Temperature must be in Kelvin because the Ideal Gas Law is based on absolute temperature. Kelvin starts at absolute zero (0 K), ensuring that the relationship between temperature and kinetic energy remains directly proportional without negative values.
The Ideal Gas Law is an approximation. It becomes less accurate at very high pressures (where gas molecules are forced close together) and very low temperatures (where intermolecular forces become significant). In these conditions, the Van der Waals equation is more appropriate.