© 2014 Pearson Education, Inc.

Chapter 11 Gases11.10 Partial Pressures

Learning Goal Use Dalton’s Law of partial pressures to calculate the total pressure of a mixture of gases.

Chapter 11 Lecture

Basic ChemistryFourth Edition

© 2014 Pearson Education, Inc.

The partial pressure of a gas

• is the pressure of each gas in a mixture

• is the pressure that gas would exert if it were by itself in the container

Partial Pressure

© 2014 Pearson Education, Inc.

Dalton’s law of partial pressures statesthat the total pressure• depends on the total number of gas

particles, not on the types of particles

• exerted by a gas mixture is the sum of the partial pressures of those gases

PT = P1 + P2 + P3 .....

Total pressure ofthe gas mixture

= Sum of the partial pressures of the gases in the mixture

Partial Pressure

© 2014 Pearson Education, Inc.

Total Pressure Equal Sum of Partial Pressures

The total pressure of two gases is the sum of their partial pressures.

© 2014 Pearson Education, Inc.

The air you breathe is a mixture of gases.

A typical mixture of gases in the air includes: N2, O2, CO2, Ar, and H2O.

Air, Mixture of Gases

© 2014 Pearson Education, Inc.

Air, Mixture of Gases

© 2014 Pearson Education, Inc.

A scuba tank contains O2 with a pressure of 0.450 atm and He at 855 mmHg. What is the total pressure in mmHg in the tank?

Learning Check

© 2014 Pearson Education, Inc.

A scuba tank contains O2 with a pressure of 0.450 atm and He at 855 mmHg. What is the total pressure in mmHg in the tank?

Step 1 Write the equation for the sum of partial pressures.

Solution

© 2014 Pearson Education, Inc.

A scuba tank contains O2 with a pressure of 0.450 atm and He at 855 mmHg. What is the total pressure in mmHg in the tank?

Step 2 Rearrange the equation to solve for the unknown pressure.

Solution

© 2014 Pearson Education, Inc.

A scuba tank contains O2 with a pressure of 0.450 atm and He at 855 mmHg. What is the total pressure in mmHg in the tank?

Step 3 Substitute known pressures into the equation and calculate the unknown pressure.

Solution

© 2014 Pearson Education, Inc.

A gas produced in the laboratory • usually contains water vapor• PT = Pwater + Pgas

• has a partial pressure obtained by subtracting the vapor pressure of water from the PT

Collecting Gas over Water

© 2014 Pearson Education, Inc.

Magnesium (Mg) reacts with HCl, to produce H2 gas.

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

•As the H2 gas is produced, it displaces some of the water in the container.

•The gas collected will be a mixture of H2 and water vapor.

Collecting Gas over Water

© 2014 Pearson Education, Inc.

We use the vapor pressure of water at the experimental temperature to determine the amount of dry H2 gas produced in the reaction.

Collecting Gas over Water

Figure 11.9 A gas from a reaction is collected by bubbling through water. Due to evaporation of water, the total pressure is equal to the partial pressure of the gas and the vapor pressure of water.

© 2014 Pearson Education, Inc.

Guide, Collecting Gas over Water

© 2014 Pearson Education, Inc.

When magnesium reacts with HCl, a volume of 0.355 L of H2 gas is collected over water at 26 °C. The vapor pressure of water at 26 °C is 25 mmHg. If the total pressure is 752 mmHg, how many moles of H2 gas were collected?

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

Learning Check

© 2014 Pearson Education, Inc.

In the following reaction 0.355 L sample of H2 gas is collected over water at 26 °C. If the total pressure is 752 mmHg, how many moles of H2 gas were collected?

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)Step 1 Obtain the vapor pressure of water.

The vapor pressure of water at 26 °C is 25 mmHg.

Solution

© 2014 Pearson Education, Inc.

In the following reaction 0.355 L sample of H2 gas is collected over water at 26 °C. If the total pressure is 752 mmHg, how many moles of H2 gas were collected?

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)Step 2 Subtract the vapor pressure from the

total P of the gas mixture to give the partial pressure of needed gas.

Solution

© 2014 Pearson Education, Inc.

In the following reaction 0.355 L sample of H2 gas is collected over water at 26 °C. If the total pressure is 752 mmHg, how many moles of H2 gas were collected?

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)Step 3 Use the ideal gas law to convert Pgas to

moles or grams of gas collected.

Solution

© 2014 Pearson Education, Inc.

Step 3 Use the ideal gas law to convert Pgas to moles or grams of gas collected.Solve for moles of H2 gas by placing the partial pressure of H2 (727 mmHg), volume of gas container (0.355 L), temperature (26 °C + 273 = 299 K), and R, using mmHg into the ideal gas equation.

Solution

© 2014 Pearson Education, Inc.

During breathing, an exchange of gases occurs in which

•oxygen from the air diffuses into the lungs and into the blood

•carbon dioxide produced in the cells is carried to the lungs to be exhaled

Chemistry Link to Health

© 2014 Pearson Education, Inc.

• In Table 11.8, partial pressures are given for the gases in air that we inhale (inspired air), air in the alveoli, and air that we exhale (expired air).

• The partial pressure of water vapor increases within the lungs because the vapor pressure of water is 47 mmHg at body temperature.

Chemistry Link to Health

© 2014 Pearson Education, Inc.

Chemistry Link to Health

© 2014 Pearson Education, Inc.

Chemistry Link to Health

© 2014 Pearson Education, Inc.

Concept Map