Today… 1)Addressing Some CURIOUS questions 2)Take-up Homework 2) Minds-On-Making Connections to Charles Law and Gay Lussacs Law 3) Combined Gas law and Dalton’s Law of Partial Pressures

Celsius and Kelvin Scales If you double the degrees Celsius, what happens to the temperature in Kelvin?

Graph of four experiments in which the same amount of gas was used and data were collected at four different pressures (P1 to P4). The solid lines represent actual data, dashed lines represent extrapolated values. All of the plots intersect at −273.15°C.

Why Kelvin and Not Celsius for gas temperature? (A) Using the Celsius temperature scale produces straight-line graphs that have three different y-intercepts. (not a direct proportion, doubling temp does not double volume) (B) Using the Kelvin temperature scale produces straight-line graphs with the same y-intercept (direct proportion, y-intercept 0, doubling temp doubles the volume) Identify the x and y-intercepts for each graph.

Combined Gas Laws and Dalton’s Law of Partial Pressures

The Combined and Ideal Gas Laws All three variables can be changed at the same time: Temperature, Pressure, and Volume. A combination of Boyle’s and Charles’s law. When the mass (amount) of gas is constant you can use:

Different forms of Equations P 1 V 1 = P 2 V 2 T 1 T 2 V 1 = P 2 V 2 T 1 P 1 T 2 V 2 = P 1 V 1 T 2 T 1 P 2 T 1 = P 1 V 1 T 2 V 2 P 2 T 2 = P 2 V 2 T 1 V 1 P 1 P 1 = P 2 V 2 T 1 T 2 V 1 P 2 = P 1 V 1 T 2 T 1 V 2 pressure and volume of a given amount of gas are inversely proportional to each other, and directly proportional to the Kelvin temperature of the gas.

The Combined Gas law Sample Problem: A weather balloon with a volume of 55.0 L is filled with hydrogen gas at a pressure of 98.5 kPa and a temperature of 13ºC. When the balloon is released, it rises to the stratosphere, where the temperature is -48ºC and the pressure is 19.7 kPa. What is the volume of the balloon under these conditions?

Plan your strategyAct on your strategy -None of them remain constant! -Which law: Combined Gas Law P 1 V 1 /T 1 = P 2 V 2 /T 2 -Isolate the variable you are looking for (in this case it is V 2 ) -Remember to convert the temperatures into Kelvin. V 2 = P 1 V 1 T 2 /P 2 T 1 Plug the numbers (and units) of the known variables into the equation. V2 = (98.5 kPa)(55.0 L)(225 K)/(286 K)(19.7 kPa) = 216 L Conclusion: The volume at -48ºC and 19.7 kPa is 216 L. Class work Worksheet: Combining Gas Laws

Dalton’s Law of Partial Pressures Gas Mixtures: In a mixture of gases, each gas exerts a pressure. The pressure exerted by a single gas in a mixture is called the Partial pressure of the gas. Dalton observed….

Mathematically this means that if a mixture is made of two gases, A and B, then: P t = P A + P B P t = Total pressure P A = Partial pressure of gas A P B = Partial pressure of gas B Law of Partial Pressures: The total pressure of a Mixture of gases is equal to the sum of the partial pressures of the component gases (if the gases do not react chemically).

When two separate gases originally at the same temperature are mixed, the temperature—and thus the average speed of the gases—does not change. Only the total number of molecules in the container increases. Therefore there are more molecules colliding with the walls of the container and, thus, the pressure is higher.

Pressure of a Gas The pressure of a gas in a mixture is also related to the amount of molecules of that gas in the mixture. Mathematically this can be stated as: is called the mole fraction of gas A.

Mole Fraction of a Gas the number of moles of gas A divided by the total number of moles of ALL gases in the mixture.

Recap of Key Formulas P T = P A + P B P T = Total pressure P A = Partial pressure of A P B = Partial pressure of B _n A_ = mole fraction A n total n A = moles of gas A n total = Total moles of gas P A = Pt x n A n total P A = partial pressure of A Pt = total pressure n A = moles of gas A n total = total moles of gas

Partial Pressure Sample Problem A mixture of 6.0 g of argon gas and 8.0 g of oxygen gas has a total pressure of 66 kPa. Calculate the partial pressure exerted by each gas.

Plan your strategyAct on your strategy -This is a partial pressure problem P T = P Ar + P O2 P Ar = P total x n Ar /n total P O2 = P total x n O2 /n total i)-Convert each mass into moles and find n total. ii) -Find the mole fraction of each gas. n A = mole fraction A n total i) n Ar = 6 g/39.95 g/mol = 0.15 mol n O2 = 8 g/32 g/mol = 0.25 mol n total = 0.25 mol mol = 0.40 mol ii)n Ar = mole fraction Ar = 0.15 mol/0.40 mol = ntotal n O2 = mole fraction O 2 = 0.25 mol/0.40 mol = ntotal -To find the partial pressure of each gas: P A = Pt x n A n total P Ar = Pt x n Ar = 66 kPa x = 25 kPa n total P O2 = Pt x n O2 = 66 kPa x = 41 kPa n total Conclusion: To check, add both partial pressures together. If their sum is equal to your total pressure (i.e. 66 kPa), then you have done the problem correctly and successfully!