Dalton's law states that each gas in a mixture of gases exerts its own pressure, behaving as if it is the only gas present. The pressure exerted by the gas is known as **partial pressure**.

The partial pressure of a gas is a measure of its concentration and is expressed in kilo pascals (kPa). The**Partial Pressure Formula** of a gas in a mixture of gases is given by

The partial pressure of a gas is a measure of its concentration and is expressed in kilo pascals (kPa). The

Where P_{A} is the partial pressure of gas A, n_{A} is the number of moles of gas A, n is the total number of moles of gas in the mixture. f_{A} is the mole fraction and P_{B} is the barometric or total pressure of the mixture of gases.

The partial pressure of a gas is an idea (rather than something we can measure). We think of the total pressure of a mixture as being composed of the pressure of each gas in the mixture. The diagram below illustrates this with three different gases (show as gases A, B, and C). When by itself in a container, each gas has a specific pressure. Those are at the top and labeled as such. Then those same amounts are all three pushed into the same container again, so now the total number of moles has increased and therefore the total pressure.

The partial pressure of a gas is an idea (rather than something we can measure). We think of the total pressure of a mixture as being composed of the pressure of each gas in the mixture. The diagram below illustrates this with three different gases (show as gases A, B, and C). When by itself in a container, each gas has a specific pressure. Those are at the top and labeled as such. Then those same amounts are all three pushed into the same container again, so now the total number of moles has increased and therefore the total pressure.

The pressure of the individual gases in the mixture is the partial pressure of that gas. The sum of the partial pressures is the total pressure.

Dalton's law of **Partial Pressure Formula** is mathematically expressed as

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Solved problems based on partial pressure are given below. ### Solved Examples

**Question 1: **To a tank containing N_{2} at 2.0atm and O_{2} at 1.0atm, we add an unknown quantity of CO_{2} until the total pressure within the tank is 4.6atm. What is the partial pressure of the CO_{2}?

** Solution: **

Dalton's law tells us that the addition of CO_{2} does not affect the partial pressures of the N_{2} and O_{2} already present in the tank. The partial pressure of N_{2} and O_{2} remain at 2.0atm, respectively and their sum is 3.0atm. If the new pressure is 4.6atm, the partial pressure of the added CO_{2} must be 1.6atm. Thus when the final pressure is 4.6atm, the partial pressures are

4.6atm = 2.0atm + 1.0atm + 1.6atm

**Question 2: **A vessel under 2.015atm pressure contains nitrogen N_{2} and water vapor H_{2}O. The partial pressure of N_{2} is 1.908atm. What is the partial pressure of the water vapor?

** Solution: **

Given

Total pressure = 2.015atm

Partial pressure of N_{2 }= 1.908atm

Partial pressure of water vapor = ?

Now we use the equation

P_{total} = P_{N2} + P_{water vapor}

2.015 = 1.908 + P_{water vapor}

P_{water vapor} = 0.107

Dalton's law tells us that the addition of CO

4.6atm = 2.0atm + 1.0atm + 1.6atm

Given

Total pressure = 2.015atm

Partial pressure of N

Partial pressure of water vapor = ?

Now we use the equation

P

2.015 = 1.908 + P

P

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