GasesChapter 5

What you need to What you need to know…know…

• PV = nRT for gas stoichiometry

• Partial pressures for kinetics and equilibrium later

• Water vapor pressure calculations

• KMT for explaining behavior of atoms/molecules in

• Gas laws (esp. graham’s law)

• Differences between real and ideal gases (spec. what causes deviations from ideal behavior)

Pressure & Pressure & TemperatureTemperature

Force per unit of area

Standard pressure is 1 atm (760 mmHg, 760 torr, 101.325 kPa)

Standard temp is 0oC (273 K)

ALL gas calculation must be done in K!!!

Gas LawsGas Laws Boyle’s: P1V1 = P2V2; as pressure goes up,

volume goes down (T & n stay constant)

Charles’: V1/T1 = V2/T2; as temp goes up, volume goes up (P & n stay constant)

Gay-Lussac’s: P1/T1 = P2/T2; as temp goes up, pressure goes up (V & n stay constant)

Avogadro’s: V1/n1 = V2/n2; as number of moles goes up, volume goes up (P & T stay constant)

More gas lawsMore gas laws Avogadro’s other law: one mole of any gas

at STP will have a volume of 22.4 L

Combined: P1V1/T1 = P2V2/T2 (n stays constant)

One final law…

Ideal Gas LawIdeal Gas LawPV = nRT or PVM = mRT or MP =

DRTWhere: P = pressure (atm)

V = volume (L)

n = number of moles (mol)

R = ideal gas constant (0.0821 L atm/mol K

T = temp (K)

M = molar mass (g/mol)

m = mass (g)

D = density (g/L)

Ideal v. Real GasesIdeal v. Real Gases• Ideal gas law is only an approximation of

gas behavior b/c there is no such thing as an ideal gas

• Most gases behave close to ideal when the pressure is below 1 atm and the temp is above 25oC

• non-polar gases behave more ideal than polar

• Smaller gases behave more ideal than larger ones

Ideal PracticeIdeal PracticeGasoline is a mixture of many hydrocarbon

compounds, but its chemical formula can be approximated as C8H18. How many liters of carbon dioxide gas are formed at 25.0oC and 712 torr when 3.80 L of liquid gasoline is burned in excess oxygen? Liquid gasoline has a density of 0.690 g/mL.

Partial PressuresPartial Pressures Dalton figured out that because gas

particles are so spread out, even when mixed with other gases they don’t really interact, so their respective pressures simply add up.

Ptotal = P1 + P2 + P3 + …

There’s also mole fraction: X1 = n1/nt

and ideal gas: P1 = n1RT/V

and partial pressure again: P1 = Pt(n1/nt)

What was all that stuff What was all that stuff again?again?

Mole fraction – ratio of moles of a specific gas to total moles of gas in a mixture

Why is pressure the only thing affected by all these gases? Because volume and temp are the same for all

gases in the same container, only pressure adds up

Partial Pressures Partial Pressures practicepractice

A mixture of 9.00 g of oxygen, 18.0 g of argon, and 25.0 g of carbon dioxide exert a pressure of 2.54 atm. What is the partial pressure of each gas in the mixture?

Water Vapor PressureWater Vapor Pressure The easiest way to collect gas in the lab is

over water, but this causes water vapor to be mixed in with the gas.

To cancel this out, the water vapor pressure must be added into the mix

To figure out the pressure of the gas collected: Patm = Pwater + Pgas

PracticePracticeHydrogen is produced by the action of sulfuric

acid on zinc metal and collected over water in a 255 mL container at 24.0oC and 718 torr. The vapor pressure of water under these conditions is 22.38 torr. How many moles of hydrogen gas are produced and how many grams of zinc react?

Kinetic Molecular Kinetic Molecular TheoryTheory

Know the 5 points of the KMT

Pressure is caused by gas particles hitting the walls of the container (how often and how hard)

Absolute temp is measure if the kinetic energy of the gas particles

Effusion and DiffusionEffusion and Diffusion Effusion is gas escaping through a tiny hole

Diffusion is random mixing of gases through space/another medium

Graham’s law inversely relates effusion rate and molecular mass (heavier gases effuse/diffuse more slowly)

r1/r2 = √(M1/M2)

where r is rate at which gas effuses (mol/time)

Effusion PracticeEffusion PracticeAt a particular temperature and pressure,

neon gas effuses at a rate of 16.0 mol/s. What is the rate of effusion for argon under the

same conditions? Under a different set of conditions, 3.0 mol of

argon effuse in 49.0 seconds. How long will it take and equal amount of helium to effuse?

van der Waalsvan der Waals van der Waals equation predict the behavior

of real gases at low temp &/or high pressure.

you do not need to know the equation, just that it implies more deviation from ideal behavior under the following conditions: large molecular mass low volume/high pressure low temperature

More PracticeMore PracticeArrange the following gases in order of

increasing deviation from ideal behavior: H2O, CH4, Ne. Justify your reasoning.