# Gases

## CHM 101

## Pressure-A gas thing!

**STP or standard temperature and pressure** is useful because we can compare volumes of gases if we have a standard temperature and pressure for comparison. The standard temperature is 0 ºC or 273.15 K and standard pressure is exactly 1 atm (same as 760 mm Hg or 760 torr or 101,325 Pa or 14.7 psi or 29.92 in Hg).

## Boyle's Law Describes the relationship between pressure and volume. This is the only basic gas law where the two variables are inversely related (P and V). As pressure increases, volume decreases and vice versa.P1V1 = P2V2 | ## Charles' Law Describes the relationship between temperature (in Kelvin) and volume. As temperature increases, volume increases. These two variables are directly related to each other.V1/T1 = V2/T2 | ## Gay-Lussac's Law Describes the relationship between temperature (in Kelvin) and pressure. As temperature increases, pressure increases. These two variables are directly related to each other. As temperature increases, so does pressure.P1/T1 = P2/T2 |

## Boyle's Law

**inversely related**(P and V). As pressure increases, volume decreases and vice versa.

P1V1 = P2V2

## Charles' Law

**directly related**to each other.

V1/T1 = V2/T2

## Dalton's Law

**Dalton's Law of Partial Pressures **states that the total pressure, **P****total**, of a mixture of gases is equal to the sum of the *partial pressures *of the gases making up the mixture. Notice the word "mixture", this law does not apply to cases where the gases react in a chemical reaction. The air we live in would apply to Dalton's Law. Because our air especially contains a lot of water here in the gulf south, the water vapor in the air is also a part of this gas mixture. We call this partial pressure of water vapor, the *vapor pressure* of water and, like all gases, that pressure is dependent on the same factors affecting gases like V, amount and especially T. Which season has more water vapor in the air: summer or winter?.

## Avogadro's Law There are several consequences of Avogadro's Law. Simply stated, the amount of a gas (in moles) is directly related to the volume as the picture above indicates. At the same conditions of pressure and temperature, equal volumes of gases contain equal numbers of particles. This has a special relationship at STP where one mole of any gas occupies 22.4 L (see diagram below). Also, if volumes and moles are directly related, we can use coefficients in chemical reactions for gases as volumes (see discussion below).V1/n1 = V2/n2 | ## Combined Gas Law Describes the relationship between temperature (in Kelvin) and volume. As temperature increases, volume increases. These two variables are directly related to each other.V1/T1 = V2/T2 | ## Ideal Gas Law The ideal gas law is an equation of state. Notice that there are no changes (no "1's" or "2's"). You have to be more careful about units however for everything to work. R = 0.0821 L atm/mol K as a proportionality constant. Therefore, volume must be in L and pressure must be in atm and, just like all of the gas laws, temperature must be in Kelvins. |

## Avogadro's Law

**directly related**to the volume as the picture above indicates. At the same conditions of pressure and temperature, equal volumes of gases contain equal numbers of particles. This has a special relationship at STP where one mole of any gas occupies 22.4 L (see diagram below). Also, if volumes and moles are directly related, we can use coefficients in chemical reactions for gases as volumes (see discussion below).

V1/n1 = V2/n2

## Combined Gas Law

**directly related**to each other.

V1/T1 = V2/T2

## Ideal Gas Law

**equation of state**. Notice that there are no changes (no "1's" or "2's"). You have to be more careful about units however for everything to work. R = 0.0821 L atm/mol K as a proportionality constant. Therefore, volume must be in L and pressure must be in atm and, just like all of the gas laws, temperature must be in Kelvins.