The Four Gas Law Variables:
Temperature, Pressure, Volume, and Moles

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I. Volume

All gases must be enclosed in a container that, if there are openings, can be sealed with no leaks. The three-dimensional space enclosed by the container walls is called volume. When the generalized variable of volume is discussed, the symbol V is used.

Volume in chemistry is usually measured in liters (symbol = L) or milliliters (symbol = mL). A liter is also called a cubic decimeter (dm3).

Other units of volume do occur such as cubic feet (cu. ft. or ft3) or cubic centimeters (cc or cm3). The main point to remember is: whatever units of volume are used, use them all the way through the problem. If you must convert from one unit to another, make sure you do it correctly.

The ChemTeam discusses various gas laws where volume is a variable. The most common way to visualize this is to imagine the container involved in the experiment has a movable wall. When the volume goes up, that wall slides out. When the volume goes down, the wall slides in. Imagine the seal of the movable wall to be perfect so no gas escapes.

If the volume is constant, then the container is made with rigid walls that cannot move. Within the limits of any experiment discussed, the walls remain fixed and the volume stays constant. In other words, the container walls never move (or break) during an experiment.

II. Temperature

All gases have a temperature, usually measured in degrees Celsius (symbol = °C). Note that Celsius is captalized since this was the name of a person (Anders Celsius). When the generalized variable of temperature is discussed, the symbol T is used.

There is another temperature scale which is very important in gas behavior. It is called the Kelvin scale (symbol = K). Note that K does not have a degree sign and Kelvin is captalized because this was a person's title (Lord Kelvin, his given name was William Thomson).

All gas law problems will be done with Kelvin temperatures. If you were to use degrees Celsius in any of your calculations, YOU WOULD BE WRONG. Your teacher may try and trip you up on this point.

You can convert between Celsius and Kelvin like this:

Kelvin = Celsius + 273.15

Often, the value of 273 is used instead of 273.15. Check with your teacher on this point. Amost all examples done by the ChemTeam will use 273. For example, 25 °C = 298 K, because 25 + 273 = 298.

I will use 273 K (zero degrees Celsius) for standard temperature.

The Kelvin temperature of a gas is directly proportional to its kinetic energy. (For example, double the Kelvin temperature, you double the kinetic energy.) This relationship will come into play from time to time.

III. Pressure

Gas pressure is created by the molecules of gas hitting the walls of the container. This concept is very important in helping you to understand gas behavior. Keep it solidly in mind. This idea of gas molecules hitting the wall will be used often. When the generalized variable of pressure is discussed, the symbol P is used.

There are three different units of pressure used in chemistry. This is an unfortunate situation, but we cannot change it. You must be able to use all three in calculations as well as being able to convert fron one to another. Here they are:

  1. atmospheres (symbol = atm)
  2. millimeters of mercury (symbol = mmHg)
  3. Pascals (symbol = Pa) or, more commonly, kiloPascals (symbol = kPa)

You will find more on pressure units in another page.

Standard pressure is defined as 1.000 atm. or 760.0 mm Hg or 101.325 kPa.

Let's pause here for a second. Make sure that you nail the values for standard pressure. You MUST have those values memorized!! Many problems will simply say "standard pressure" and you have to already know the value. Here they are again:

Pressure UnitStandard Value
atm1.000
mmHg760.0
kPa101.325

Standard temperature and pressure is a very common phrase in chemistry, so common it has been abbreviated to STP. What the ChemTeam will use as STP is actually called standard ambient temperature and pressure (STAP), but the difference between the two is unimportant at this stage of your chemistry training.

There is no such thing as standard volume, although you will probably learn about molar volume in your class.

IV. Amount of Gas

The amount of gas present is measured in moles (symbol = mol) or in grams (symbol = g or gm). Typically, if grams are used, you will need to convert to moles at some point. When the generalized variable of amount in moles is discussed, the letter "n" is used as the symbol (note: the letter is in lowercase. The others above are all caps.).


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