Gas Pressure Calculator (Combined Gas Law)
Final Pressure (atm)
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Final Pressure (kPa)
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Final Pressure (psi)
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Understanding the Combined Gas Law
The combined gas law relates pressure, volume, and temperature of a gas: (P₁V₁)/T₁ = (P₂V₂)/T₂. It combines Boyle's law (P inversely proportional to V at constant T), Charles's law (V proportional to T at constant P), and Gay-Lussac's law (P proportional to T at constant V).
Temperature must be in Kelvin (K = °C + 273.15). Using Celsius or Fahrenheit will give incorrect results because the gas laws require an absolute temperature scale where zero represents zero molecular motion.
This law applies to ideal gases — a good approximation for real gases at moderate temperatures and pressures. It's used in chemistry, meteorology, engineering, and scuba diving (predicting air tank behavior at different depths).
Frequently Asked Questions
What is the combined gas law?
(P₁V₁)/T₁ = (P₂V₂)/T₂. It relates initial and final states of pressure, volume, and temperature for a fixed amount of gas.
Why must temperature be in Kelvin?
Gas laws require absolute temperature. Kelvin starts at absolute zero (-273.15°C) where molecular motion theoretically stops. Celsius can give negative values that produce incorrect results.
What is an ideal gas?
A theoretical gas where molecules have no volume and no intermolecular forces. Real gases approximate this at high temperatures and low pressures.
What is standard temperature and pressure (STP)?
STP is 0°C (273.15 K) and 1 atm (101.325 kPa). At STP, one mole of ideal gas occupies 22.414 liters.