Chapter 11 Molecular Composition of Gases

11.1 Volume-Mass Relationship Early 1800’s Gay-Lussac notices something interesting… 2L H 2 + 1L O 2 → 2L H 2 O 2mL H 2 + 1mL O 2 → 2mL H 2 O 600L H L O 2 → 600L H 2 O There is a 2:1:2 relationship! Just like in the balanced equn 2 H 2 + O 2 → 2 H 2 O Gay-Lussac’s Law of Combining Volumes of Gases: At constant temperature and pressure, the volumes of gaseous reactants and products can be expressed as ratios of small whole numbers.

Avogadro’s Law 1811 : Equal volumes of gases at the same temperature and pressure contain an equal # of particles/moles.

REMEMBER?!? Molar Volume of a gas is 22.4 L / mol No matter what the gas is as long as it’s at STP EX: At STP, what is the volume of mols of nitrogen gas?

11.2 The Ideal Gas Law Ideal Gas Law: The mathematical relationship among pressure, volume, temperature and the # of moles of a gas. Describes the behavior of an IDEAL gas. A gas described by the Kinetic-Molecular theory. PV= n RT P- pressure V- volume N - moleR- universal gas constant T- temperature

Universal gas constant (R) It’s value depends on the Pressure units. P. 342 Numerical ValueUnits atm - L/ mol - K Pa – m 3 / mol – K kPa – L/ mol- K J/ mol – K Bar- L/ mol - K 62.4mmHg- L/ mol- K torr- L/ mol- K

Practice Problem How many moles of gas at 100 °C does it take to fill a 2.5L flask at 1.50 atm? Convert 100 °C to K … = 373K PV= n RT – (1.50 atm) (2.5L) = n ( atm-L/mol-K) (373K) = moles

More practice What pressure, in atm, is exerted by mol of H 2 gas in a 4.08L container at 35°C? A sample that contains 4.35 mol of a gas at 250 K has a pressure of atm. What is the volume?

Finding Molar Mass or Density from the Ideal Gas Law When you are given GRAMS of a gas… you must convert to moles to use the Ideal Gas Law. Use M for molar mass, m for mass & the equn: – PV = mRT/M or M = mRT/PV Density (D) is mass/volume. Can use the Ideal Gas Law to solve for density. – D = MP/RT remember… M = molar mass

Practice Problems What is the density of 10.5g of argon gas at a pressure of 551 torr & a temperature of 25 °C? What is the molar mass of a gas if 0.427g of the gas occupies a volume of 125 mL at 20.0 °C & atm?

11.3 Stoichiometry of Gases a review By using what we’ve learned from Gay-Lussac and Avogadro… we can calculate Volume- Volume and Volume- Mass problems. When converting Vol- Vol… use the coefficients When converting Mass- Vol… use 22.4L/ mol, mole to mole, mole to mass.

Practice Problems What volume of O 2 is needed to react completely with 0.699L of CO to form CO STP? How many grams of water can be produced from the complete reaction of 3.77L of O 2, at STP, with H 2 ?

11.4 Effusion and Diffusion REMEMBER… Diffusion: Gases can spread out and mix. The mixture will be spontaneous & uniform. Effusion: When a gas escapes a container though small holes. Now the RATE at which a gas diffuses/effuses can be calculated…

Graham’s Law of Effusion Scottish Chemist, Thomas Graham studied the rates of diffusion and effusion. Graham’s Law: the rates of effusion at the same temperature and pressure are inversely proportional to the square root of their molar masses.

More massive particles move at a slower velocity than a less massive particle. DiffusionEffusion