Chapter 9Gases: Their Properties and Behavior

Gas mixtures are homogeneous and compressible.

Pressure:Unit area

Force

4

If there is a differencein pressure acrossthe eardrum membrane,the membrane will bepushed out – what we commonly call a “popped eardrum.”

Barometer

Patorr

mm Hgatmbar

Units

(exact)

Conversions

1 torr = 1 mm Hg

1 atm = 101 325 Pa

(exact)1 atm = 760 mm Hg

(exact)1 bar = 1 x 105 Pa

Ideal Gas: A gas whose behavior follows the gas laws exactly.

The physical properties of a gas can be defined by four variables:

P pressure

T temperature (calculation must be in Kelvin)

V volume

n number of moles

Boyle’s Law

VP1

(constant n and T)

Boyle’s Law

VP

1(constant n and T)

PinitialVinitial = PfinalVfinal

Boyle’s Law

VP

1(constant n and T)

PV = k

Boyle’s Law and Divingsince water is denser than

air, for each 10 m you dive below the surface, the pressure on your lungs increases 1 atmat 20 m the total pressure

is 3 atm

12

if your tank contained air at 1 atm pressure you would not be able to inhale it into your lungs

Examples

Calculate the volume of a sample of a gas at 5.75 atm if it occupies 5.14 L at 2.49 atm. (Assume constant temperature)

Charles’ LawV T (constant n and P)

= kT

V

=Tfinal

Vfinal

Tinitial

Vinitial

Charles’s Law

V T (constant n and P)

= kT

V

=Tfinal

Vfinal

Tinitial

Vinitial

Charles’s Law

V T (constant n and P)

ExamplesA sample of argon gas that originally

occupied 14.6 L at 25.0oC was heated to 50.0oC at constant pressure. What is its new volume?

Avogadro’s Law

V n (constant T and P)

= kn

V

=nfinal

Vfinal

ninitial

Vinitial

ExamplesConsider two samples of nitrogen gas

(composed N2 molecules). Sample 1 contains 1.5 mol of N2 and has a volume of 36.7 L at 25.0oC and 1.0 atm. Sample 2 has a volume of 16.0 L at 25.0oC and 1.0 atm. Calculate the number moles of N2 in sample 2

=Avogadro’s Law:

PinitialVinitial = PfinalVfinalBoyle’s Law:

nfinal

Vfinal

ninitial

Vinitial

Charles’ Law: =Tfinal

Vfinal

Tinitial

Vinitial

SummaryIf the systems is disturbed by one of the four variables: O, T, n then considerthe following changes

Combine Gas Law is an expression obtained by mathematically

combining Boyle’s and Charles’ law P1V1 = P2V2 @ constant n

T1 T2

can predict P, V or T when condition is changed

ExamplesSuppose we have a 0.240 mol sample of

ammonia gas at 25.0oC with a volume of 3.5 L at a pressure of 1.68 atm. The gas compressed to a volume of 1.35 L at 25.0oC. Use the combined gas law to calculate the final pressure.

PV = nRT

Standard Temperature and Pressure (STP) for Gases

Ideal Gas Law:

P = 1 atm

T = 0 °C (273.15 K)

R is the gas constant and is the same for all gases.

R = 0.082058K mol

L atm

ExamplesWhat volume is occupied by 25.7 g of carbon dioxide gas

at 25.0oC and 371 torr?A 0.250 mol sample of argon gas has a volume of 9.00L at

a pressure of 875 mmHg. What is the temperature (in oC) of the gas?

Determine the molar mass of a gas with a density of 1.905 g/L at 80.0oC

= 22.41 LV =P

nRT=

What is the volume of 1 mol of gas at STP?

(1 atm)

(1 mol) 0.08206K mol

L atm(273.15 K)

examplesA sample of nitrogen gas has a volume of

1.75 L at STP. How many moles of N2 are present?

What is the volume of 12.5 g of an methane (CH4) gas at STP