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Gas Laws

Explore the principles of ideal gas behavior, usage of the ideal gas law in gas sample calculations, and applications of gas laws in determining gas properties and molar volumes. Includes explanations and examples. Ideal text for understanding gas behavior and calculations.

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Gas Laws

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  1. Gas Laws • The ideal gas law is used to describe the behavior of an idealgas. • Idealgas:hypothetical gas that obeys kinetic molecular theory and the ideal gas law Note: You should plan on writing the definition of an ideal gas on your unit 6 exam!

  2. Gas Laws • The ideal gas law is used in calculations for a specific sample of gas that has a constant T, P, V, and n. • i.e. no changes are being made to the sample of gas • If you know 3 of the 4 variables, you can calculate the other using the ideal gas law.

  3. Gas Laws Example: Calculate the volume of 1.00 mol of an ideal gas at 1.00 atm and 0.oC.

  4. Gas Laws • The temperature and pressure used in the previous problem are commonly used to report the properties of gases. • Standard Temperature and Pressure (STP): • 0.oC and 1 atm. • KNOW THIS !!

  5. Gas Laws • Molar volume: • the volume one mol of a gas occupies (L/mole) • At STP, one mole of an ideal gas has a molar volume of 22.4 L: • 22.4 L 1 mol

  6. Gas Laws • The ideal gas law applies only to ideal gases. • Does not always accurately describe real gases • The molar volumes for many real gases at STP differ slightly from 22.4 L/mol. • In most cases, the differences between ideal gas behavior and real gas behavior is so small that we can ignore it.

  7. Gas Laws Example: A weather balloon contains 4.75 moles of He gas. What volume does the gas occupy at an altitude of 4300 m if the temperature is 0.oC and the pressure is 0.595 atm?

  8. Gas Laws Example: A used 425 mL aerosol can contains 0.761 g of propane gas (as a propellant). Calculate the pressure (in torr) in the can if it is accidentally heated to 395oC. (Warning: Don’t do this!!)

  9. Gas Laws Example: A tire with an interior volume of 3.50 L contains 0.357 mol of air at a pressure of 2.49 atm. What is the temperature of the air in the tire in oC?

  10. Gas Laws • The ideal gas law is useful in determining the properties of a specific sample of gas at constant T, P, V, and n. • We often need to know how a change in one (or more) properties impacts the other properties for a sample of a gas.

  11. Gas Laws • Combined Gas Law: P1V1 = P2V2 T1 T2 This equation is true when the number of moles of a gas is constant. You cannot apply the combined gas law when a chemical reaction occurs!

  12. Gas Laws Special cases for the combined gas law: • At constant temperature (T1 = T2), P1V1 = P2V2 • At constant volume (V1 = V2), P1= P2 T1 T2 • At constant Pressure (P1 = P2), V1 = V2 T1 T2

  13. Gas Laws Example: A helium-filled balloon occupies 6.00 L at 25oC and 0.989 atm. What volume will the balloon occupy on top of Pike’s Peak if the pressure is 0.605 atm and the temperature is 19oC?

  14. Gas Laws Example: Suppose a used aerosol can contains a gas at 0.989 atm at 23oC. If this can is heated to 425oC, what is the pressure inside the can (assuming that the can does not rupture)?

  15. Gas Laws--More Applications • The ideal gas equation can be used to determine either the density or the molar mass of a gas. d = P M RT where d = density P = pressure T = temperature in K R = gas constant M = molar mass

  16. Gas Laws--More Applications • The density of a gas depends on: • pressure • temperature • molar mass • At constant temperature and pressure the densities of gas samples are directly proportional to their molar masses: • d = PM • RT

  17. Gas Laws – More Applications Example: Which of the following gases will be more dense than dry air (assuming an average molar mass of 29 g/mol) at 1 atm and 25oC? methane carbon dioxide helium fluorine

  18. d = P M • RT Gas Laws--More Applications • If molar mass and pressure are held constant, then the density of the gas will decrease with increasing temperature. • Hot air rises because density is inversely proportional to temperature.

  19. Gas Laws--More Applications Example: What is the density of helium gas at 1.00 atm and 25oC?

  20. Gas Laws--More Applications Example: What is the average molar mass of an unknown substance if it has a density of 1.42 g/L at 21oC and 750.0 torr?

  21. Gas Laws--More Applications • Understanding the properties of gases is important because gases are often the reactants or products in a chemical reaction. • Often need to calculate the volume or pressure of gas produced or consumed during a reaction

  22. grams A grams A Moles A Moles A Molar mass Molar mass PV = nRT Molar ratio Molar ratio grams B grams B Moles B Moles B Molar mass Molar mass PV = nRT Gas Laws--More Applications PA, VA, TA PB, VB, TB

  23. Gas Laws--More Applications Example:The air bag in a car is inflated by nitrogen gas formed by the decomposition of NaN3: 2 NaN3(s)  2 Na (s) + 3 N2 (g) If an inflated air bag has a volume of 36 L and is to be filled with N2 gas at a pressure of 1.15 atm at a temperature of 26oC, how many grams of NaN3 must be decomposed?

  24. Gas Laws--More Applications Example: How many mL of oxygen gas can be collected at STP when 1.00 g of KClO3 decomposes: 2 KClO3 (s)  2 KCl (s) + 3 O2 (g)

  25. Gas Laws – More Applications Example: What volume of CO2 at 125oC and 1.15 atm will be produced by the combustion of 1.00 L of C2H6 at 25oC and 1.00 atm? 2 C2H6 (g) + 7 O2 (g)  4 CO2 (g) + 6 H2O (g)

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