What is the Combined Gas Law?

The Combined Gas Law merges Boyle's Law, Charles's Law, and Gay-Lussac's Law into one equation: (P1×V1)/T1 = (P2×V2)/T2. It describes how pressure (P), volume (V), and temperature (T) of a fixed amount of gas are related. This law is useful when all three properties change simultaneously, unlike the individual gas laws which hold one variable constant.

When should I use the Combined Gas Law instead of individual gas laws?

Use the Combined Gas Law when two or more gas properties (pressure, volume, temperature) change simultaneously. If only pressure and volume change (constant temperature), use Boyle's Law. If only volume and temperature change (constant pressure), use Charles's Law. If only pressure and temperature change (constant volume), use Gay-Lussac's Law. The Combined Gas Law is more versatile for complex scenarios.

What units should I use for the Combined Gas Law?

Temperature must always be in Kelvin (absolute scale). Pressure can be in any units (atm, Pa, mmHg, psi) as long as P1 and P2 use the same units. Similarly, volume can be in any units (L, mL, m³) as long as V1 and V2 match. The ratio format of the equation cancels out units, but consistency is essential.

Does the Combined Gas Law work for any amount of gas?

Yes, but the amount of gas (number of moles) must remain constant. The Combined Gas Law applies to a fixed quantity of gas undergoing changes in pressure, volume, or temperature. If gas is added or removed, you need to use the Ideal Gas Law (PV = nRT) instead, which includes moles (n) as a variable.

How is the Combined Gas Law different from the Ideal Gas Law?

The Combined Gas Law compares two states of the same gas sample: (P1V1)/T1 = (P2V2)/T2. The Ideal Gas Law (PV = nRT) describes a single state and includes the number of moles and gas constant. When the amount of gas is constant, you can derive the Combined Gas Law from the Ideal Gas Law by setting the two states equal.

Math

Combined Gas Law Calculator

Calculate pressure, volume, and temperature relationships using the Combined Gas Law.

Solve For

Initial Pressure (P1)

Initial Volume (V1)

Initial Temp (T1) [K]

Final Pressure (P2)

Final Volume (V2)

Final Temp (T2) [K]

How to Use This Calculator

Enter Initial State Variables

Input the initial pressure (P1), volume (V1), and temperature (T1) of your gas. Ensure temperature is in Kelvin. For pressure and volume, note which units you're using and stay consistent.

Enter Known Final State Variables

Input two of the three final state variables (P2, V2, or T2). The calculator will solve for the unknown third variable using the Combined Gas Law equation.

Calculate and Verify

Click calculate to find your missing variable. Review the results to ensure they make physical sense - for example, if both pressure and temperature increase, volume should also change predictably. The calculator will show the step-by-step calculation.