Combined Gas Law

Basic Concept

Combined Gas Law

The Combined Gas Law is the expression obtained from mathematically combining Boyle's, Charles's and Gay-Lussac's laws.

The mathematical form of the Combined Gas Law is:

(P1 x V1) / T1 = (P2  x V2 ) / T2   

When any five of the six quantities in the equation are known, the sixth can be calculated. For example, we've known P1, V1, T1, P2  and V2 , the T2  can be:

T2  = P2  x V2  x T1 / (P1 x V1)


The force exerted per unit area of surface, typical pressure units are ATM, mmHg and kPa.


The measurement of space taken by a substance, it is length cubed, typical units are L, mL and m3.


A measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale. Typical units are K, F and C.


The mass of the object divided by its volume. Typical units are g/mL and kg/m3.

Formula Weight / Molecular Weight (MW)

The formula weight of a compound is the sum of all the atomic weights of the elements present in the formula of the compound. Some text also refers it to formula mass. Typical unit is g/mol.

Mass and Mole

Mass is the amount of a substance in grams, also called weight.

Gas Constant (R)

The constant that appears in the ideal gas equation (PV=nRT). It is usually expressed as 0.08206 L x atm/K x mol or 8.314 J/K x mol.


The number of moles of solute dissolved in one kilogram of solvent.


The number of moles of solute in one liter of solution.

Feature Overview

With six quantities in (P1 x V1) / T1 = (P2  x V2 ) / T2  equation, given any five, the sixth one can be calculated based on the Combined Gas Law.

In addition to the problem solving module, the "Show Work" is also displayed along with the solution to illustrate the step-by-step guide in how your problem has been solved.

User Instructions

This module is to compute the final pressure, volume or temperature of a gas in the known initial pressure, volume and temperature and any other two final quantities.

This is one step process, enter the known data and press Calculate to output the unknowns.

1. Select Combined Gas Law link from the front page or Combined Gas tab from the Gases module. The Input and Output screen appears.

2. In the Input area, enter the three known quantities with a proper significant figure. Select the units associated with the input.

3. Click Calculate to output the answer.

4. The Show Work area on the right shows you step-by-step how your problem has been solved.

To start a new problem, click Reset. All Input fields will be cleared. Follow Step 1-3 again.