Combined Gas Law is built for P₁V₁/T₁ = P₂V₂/T₂ — fast, free, and private. Type P₁ (atm), V₁ (L) and T₁ (K) into Combined Gas Law; each keystroke re-runs the math. Every Combined Gas Law answer can be reproduced by hand. Combined Gas Law works for one check or many. Combined Gas Law is privacy-first: Science inputs stay offline. Combined Gas Law handles combined gas law calculator pressure volume temperature the same way as free online combined gas law calculator — one of the Science tools on CalcProMaster. The labels in Combined Gas Law match the terms in its result. Combined Gas Law answers the next Science question you meet.
What the defaults give
10 L at 1 atm and 273 K becomes V₂ = 5.49 L at 2 atm and 300 K.
Doubling the pressure would halve the volume to 5.0 L; the 27 K rise adds about 10% back.
The formula
P₁V₁ ÷ T₁ = P₂V₂ ÷ T₂
V₂ = P₁ × V₁ × T₂ ÷ (P₂ × T₁)
It combines Boyle law (pressure and volume inverse at constant temperature), Charles law (volume and temperature direct at constant pressure) and Gay-Lussac law (pressure and temperature direct at constant volume) into one statement: for a fixed amount of gas, PV/T is constant.
Worked examples
10 L, 1 → 2 atm, 273 → 300 K → 5.49 L
Same but pressure unchanged → 10.99 L
Same but temperature unchanged → 5.00 L
Same but starting at 20 L → 10.99 L
Start at 2 atm, end at 2 atm, 10 L, 300 → 300 K → 10.00 L
Start at 2 atm, end at 1 atm, 273 → 273 K → 20.00 L
Temperature must be absolute
Kelvin, always. The relationship is proportional only from absolute zero, and using Celsius makes the ratio wrong by hundreds of percent: 300 K is not 27 °C in a ratio, it is 300. Convert with K = °C + 273.15 before entering anything, and remember that doubling 300 K to 600 K doubles the volume at constant pressure, while doubling 27 °C to 54 °C does not.
When it applies, and when it does not
The law assumes a fixed mass of gas behaving ideally. Compress a real gas hard and the molecules take up a meaningful fraction of the volume, and intermolecular attraction matters — a real gas at high pressure has a larger volume than the ideal prediction. Mild conditions, near atmospheric pressure and room temperature, are where the combined law is accurate enough for practical work.
It also assumes no gas escapes. A leaking vessel or a reaction that changes the number of moles breaks the identity, and the answer will be silently wrong. For a reaction, use the ideal gas law with the mole count, not the combined law, because the constant PV/T is only constant for a fixed amount of gas.
Frequently Asked Questions
What does the combined gas law calculate?
The missing variable when pressure, volume and temperature change between two states of a fixed mass of gas, using P₁V₁/T₁ = P₂V₂/T₂.
Why is V₂ 5.49 L and not 5.0 L?
Because the temperature rises from 273 K to 300 K while the pressure doubles. The pressure halves the volume, the warming adds about 10% back.
Can I use Celsius in this formula?
No. Temperature must be in kelvin, because the relationship is proportional from absolute zero. Convert with K = °C + 273.15 first.
What happens if the temperature stays the same?
The law reduces to Boyle law, P₁V₁ = P₂V₂. At 2 atm from 1 atm, 10 L becomes exactly 5.00 L.
When does the ideal gas assumption fail?
At high pressure or low temperature, where molecular volume and attraction matter. The law is most accurate near atmospheric pressure and ordinary temperatures.
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