**Dalton's Law of Partial Pressure**

While studying the properties of air, John Dalton observed that the *total
pressure of a mixture of gases equals the sum of the pressures that each would exert if it
were present alone. *The* * pressure exerted by a particular
component of a mixture of gases is called the **partial pressure** of that
gas, and Dalton's observation is known as **Dalton's law of partial pressures.**

If we let P_{t} be the total pressure of the mixture, and P_{1}, P_{2},
P_{3}, etc, be the partial pressures of the gases in a mixture, we can write
Dalton's law in the following manner:

P_{t }= P_{1} + P_{2} + P_{3} + ...

**Partial Pressure and Mole Fractions**

Since each gas in a mixture behaves independently, we can relate the amount of a given gas in a mixture to its partial pressure. For an ideal gas P = nRT/V, so we can write

P_{1}n_{1}~~RT/V~~P_{1}n_{1}--- = ------- or --- = --- = X_{1}P_{t}_{ }n_{t}~~RT/V~~P_{t}n_{t}

The ratio n_{1}/n_{t} is called the mole fraction of gas 1, which
we denote X_{1}.

If we rearrange the equation above we get:

**
P _{1} =X_{1}P_{t}_{ }**

**
The partial pressure of gas 1, P _{1} is equal to its mole fraction, X_{1} times the total pressure, P_{t}.**