# The Combined Gas Law

The combining of these three gas laws is called the combined gas law. Gas at room temperature has an absolute pressure of 1 bar.

When Avogadro’s law is added to the combined gas law, the ideal gas law results. It’s a shame that we can’t pin the discovery of the combined gas law on any one person. It is a combination of the other gas laws that works when everything except temperature, pressure, and volume are held constant.

There are a couple of common equations for writing the combined gas law. This is a classic law relating Boyle’s law and Charles’ law.:

PV/T = k

where P = pressure, V = volume, T = absolute temperature (Kelvin), and k = constant.

The constant k is a true constant only if the number of moles of the gas doesn’t change.

The common formula for the combined gas law relates “before” and “after”:

P1V1 / T1 = P2V2 / T2

## Example

At STP, the volume of a gas at a specified pressure and temperature is calculated by using the following equation: V = 0.

In order to solve the problem, you first need to figure out what formula you should be using. At STP, there are two kinds of questions. You’re dealing with an “after” question because you’re looking for answers to problems that have already occurred, or the question is talking about problems that have already been identified.

In addition to knowing what the STP of the air is, you also need to understand STP. The law of conservation of energy uses absolute temperature. You must convert 25 degrees Celsius to the Kelvin scale. That gives you a degree of zero point zero zero nine zero four zero Kelvin.

At this point, you know how to find the median, average, mode, and median absolute deviation (MAD) for your data. A common mistake some people make when they’re new to this kind of problem is thinking that the two numbers have to be equal. Identifying the variables is essential to creating a model.

P1 = 745.0 mm Hg
V1 = 2.00 L
T1 = 298 K
P2 = 760.0 mm Hg
V2 = x (the unknown you’re solving for)
T2 = 273 K

The formula and set it up to solve for the unknown “x”, which is V2.

P1V1 / T1 = P2V2 / T2

Cross-multiply to clear the fractions:

P1V1T2 = P2V2T1

Divide to isolate V2:

V2 = (P1V1T2) / (P2T1)

Plug in the numbers and solve for V2:

V2 = (745.0 mm Hg · 2.00 L · 273 K) / (760 mm Hg · 298 K)
V2 = 1.796 L

Report the result using the correct number of significant figures:

V2 = 1.80 L