PV = nRT
Choose what to solve for, fill the other three
Uses R = 0.08206 L·atm·mol⁻¹·K⁻¹ (workbook reference). Methodology & sources →
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View on Amazon →Using the Ideal Gas Law
The ideal gas law links pressure, volume, amount and temperature of a gas. Always convert temperature to kelvin and keep pressure and volume in units that match your value of R. This tool converts everything to atm, litres and kelvin before solving, then reports the answer in your chosen pressure and volume units. For two-state problems (a gas changing conditions) the combined gas law P₁V₁/T₁ = P₂V₂/T₂ is quicker; read more in The Ideal Gas Law Explained.
Worked Example — Finding Mass from P, V, and T
Question: Krypton gas in an 18.5 L cylinder exerts a pressure of 11.2 atm at 28.2°C. How many grams of Kr are present?
Step 1 — convert temperature to kelvin: T = 28.2 + 273.15 = 301.35 K
Step 2 — solve PV = nRT for n: n = PV ÷ RT = (11.2 × 18.5) ÷ (0.08206 × 301.35) = 8.38 mol
Step 3 — convert moles to mass using Kr's molar mass (83.80 g/mol): m = 8.38 × 83.80 = 702 g
Answer: 702 g of Kr.
Common Mistakes
- Forgetting the °C → K conversion. The ideal gas law requires absolute temperature. Using 28.2 instead of 301.35 gives an answer roughly 10× too large.
- R value doesn't match the pressure unit. R = 0.08206 L·atm/(mol·K) only works with pressure in atm and volume in L. If pressure is given in kPa, either convert to atm first or use R = 8.314 J/(mol·K) with SI units throughout.
- Mixing volume units. If V is in mL, convert to L before using R = 0.08206 — a factor of 1000 error is easy to miss.
Frequently Asked Questions
With atm and litres, R = 0.08206 L·atm·mol⁻¹·K⁻¹. This calculator converts your inputs to those units and uses that value, so you can enter kPa or mmHg and still get the right answer.
Yes — the law needs absolute temperature. Add 273.15 to a Celsius value. Choose °C above and the calculator does it for you.
Use R = 0.08206 L·atm·mol⁻¹·K⁻¹ when pressure is in atm and volume is in litres (the most common case in intro chemistry). Use R = 8.314 J·mol⁻¹·K⁻¹ when working entirely in SI units (pascals, cubic metres). This calculator handles the conversion for you — just enter your given units.
Convert first: 1 atm = 101.325 kPa = 760 mmHg = 760 Torr. This calculator accepts multiple pressure units directly and converts internally, so you don't need to do this by hand.