John Dalton 1766 1844 Developed the Law of

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John Dalton (1766 – 1844) - Developed the Law of Partial Pressures (abt. 1801)

John Dalton (1766 – 1844) - Developed the Law of Partial Pressures (abt. 1801)

Partial Pressures Dalton said that molecules of different gases in a mixture act independently

Partial Pressures Dalton said that molecules of different gases in a mixture act independently of one another in exerting a force on the wall of the container. In other words, the frequency of collisions of one gas is independent of the presence of other gases The PRESSURE created by the collision of one gas in a mixture of gases is called the Partial Pressure of the gas

Summary Dalton found that the total pressure of mixed gases is equal to the

Summary Dalton found that the total pressure of mixed gases is equal to the sum of their individual pressures (provided the gases do not react). 100 k. Pa 50 k. Pa 150 k. Pa Note: all of these + = volumes are the 1 L oxygen 1 L nitrogen 1 L mixed gas same This works according to the KMT because at the same temperature molecules of different gases have the same energy. It doesn’t matter if the molecules are O 2 or H 2. Both collide with the container or other molecules with the same force.

Partial Pressure Problem A diving tank contains a mixture of oxygen and nitrogen. The

Partial Pressure Problem A diving tank contains a mixture of oxygen and nitrogen. The pressure exerted by these gases Is 7. 2 atm. The partial pressure of the nitrogen Is 5. 76 atm. What is the pressure exerted by The oxygen in atm & k. Pa? Ptot = PO 2 + PN 2 7. 2 atm = P 02 + 5. 76 atm PO 2 = 1. 4 atm x 101. 3 k. Pa = 145. 87 = 150 k. Pa 1 atm

Vapor Pressure Defined • Vapor pressure is the pressure exerted by a vapor. E.

Vapor Pressure Defined • Vapor pressure is the pressure exerted by a vapor. E. g. the H 2 O(g) in a sealed container. Eventually the air above the water is filled with vapor pushing down. As temperature , more molecules fill the air, and vapor pressure . • Yet, molecules both leave and join the surface, so vapor pressure also pushes molecules up.

Measuring Vapor Pressure • When the vapor pressure is equal to the atmospheric pressure

Measuring Vapor Pressure • When the vapor pressure is equal to the atmospheric pressure (Patm), the push out is enough to overcome Patm and boiling occurs. • Thus, water will boil at a temperature below 100 °C if the atmospheric pressure is reduced.

Collecting gases over water • Many times gases are collected over H 2 O

Collecting gases over water • Many times gases are collected over H 2 O • Water Vapor can be determined from a chart of known values at given temperatures • The water vapor pressure must be subtracted from the total pressure (to get the pressure of the dry gas).

Sample calculation An unknown gas was collected over 28°C H 2 O. If the

Sample calculation An unknown gas was collected over 28°C H 2 O. If the Patm=102. 9 k. Pa, determine the pressure of the unknown gas. Ptotal = PH 2 O + Pgas 102. 9 k. Pa = 28. 3 mm Hg + Pgas Convert to get same units! 28. 3 mm Hg x 101. 3 k. Pa = 3. 77 k. Pa 760 mm Hg 102. 9 k. Pa - 3. 77 k. Pa = 99. 1 k. Pa

Using Percents • Dalton’s law can be applied to molar masses as well. •

Using Percents • Dalton’s law can be applied to molar masses as well. • Because the partial pressure of each gas can be added to find the total, the percent of each gas must add up to 100% • The partial molar mass of each gas adds up to the total molar mass of the mixture.

Sample Problem A study of the effects of certain gases on plant growth requires

Sample Problem A study of the effects of certain gases on plant growth requires a synthetic atmosphere composed of 1. 5% CO 2, 18. 0 % O 2, and 80. 5 %Ar. What is the molar mass of the synthetic atmosphere? Convert the percent to decimal form! 1. 5/100 =. 015 18. 0/100 =. 180 80. 5/100 =. 805 Molar mass CO 2 = 44. 0 g x. 015 = 0. 66 g Molar mass O 2 = 32. 0 g x. 180 = 5. 76 g Molar mass Ar = 39. 9 g x. 805 = 32. 1 g Add partial molar masses together. 66 + 5. 76 + 32. 1 = 38. 5 g