LOH Ideal Gas Law lesson in science .ppt

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1. Ideal Gas Law

2. Describing a sample of a gas
•4 variables are needed to completely describe
a sample of a gas:
•Temperature
•Pressure
•Volume
•Amount (number of moles) of gas

3. Equation of State
•An equation relating the macroscopic
variables that describe some type of matter.
•The ideal gas law is an equation of state for
gases.

4. Boyle’s Law Graphs
Pressure vs. Volume
Volume vs. 1/Pressure

5. Charles’ Law Graph

6. Gay-Lussac’s Law Graph

7. Avogadro’s Law

8. Recall
Boyle’s LawCharles’ LawAvogadro’s
Law
V  1/PV  T (Kelvin)V  n
Constant T, nConstant P, nConstant T, P
So V  1/P X n X T

9. Ideal Gas Law
•To turn a proportionality into an equation,
insert a constant: V = RnT/P
•Or multiply both sides by P:
•PV = nRT where R is the ideal gas law
constant. If three of the variables are known,
the 4
th
can be determined.
•The units of R depend on the units used for P,
T, and V.

10. Units of R
•Two common values of R:
•0.08206 LiterAtm
•8.314 Joules or 8.314 LiterKPa
MoleK
MoleK MoleK

11. Problem-Solving
•Most commonly used value of R:
•0.08206 LiterAtm
•Note:
•Pressure must be in atm
•Volume must be in liters
•Temperature must be in Kelvins
MoleK

12. Ideal Gas Law Problems
•What pressure is exerted by 0.75 moles of a
gas at 25C in a container with a volume of 1.5
L?
•Find the volume of 0.85 moles of gas at a
pressure of 520 torr and a temperature of
15C.
•How many moles of gas are present in a
sample at 700 torr, 333C, and occupying a
volume of 452 mL?

13. Extensions of Ideal Gas Law Problems
•PV = nRT
•n = mass
•Density = mass
formula mass
volume

14. Memorize the three fundamental
equations.
•Substitute and rearrange as problem
demands.
•PV = nRT or PV = massRT
•Rearrange to solve for mass or density
•(Pformula mass) = mass/volume = density
formula mass
volume
RT

15. Formula mass (Identity) of gas
•Formula mass = massRT
•Or formula mass = densityRT
PV
P

16. Density of a gas
•At STP, density of a gas = molar mass/22.4 L
•This is an easy relationship but it is only true
at STP!!!

17. Dalton’s Law of Partial Pressures

18. Dalton’s Law of Partial Pressures
•For a mixture of gases:
•P
tot = P
1 + P
2 + P
3 + …
•The total pressure of a gas mixture is the sum
of the partial pressures of the component
gases. The pressure exerted by each gas in an
unreactive mixture is independent of the other
gases in the mixture.

19. Addition or Subtraction Problems
•Three gases, He, N
2, and Ar are present in a
gas mixture. The partial pressure of He is 360
torr, of N
2 is 400 torr, and of Ar is 250 torr.
Find the total pressure.
•Ar is added to the above mixture until the
total pressure is 1200 torr. Find the new
pressure of the Argon.

20. Collecting Gas by Water Displacement
When the water
levels inside and
outside the inverted
container are the
same, the total
pressure inside the
container above the
water is equal to the
atomspheric
pressure.

21. Sometimes gases are collected by
water displacement
C
2H
2(g) + H
2O(g)

22. Line up the water levels!

23. P
tot = P
gas + P
water
•The volume of gas collects above the water,
but some of the water molecules evaporate
and go into the gas phase as well.
•The vapor pressure of water depends only on
the temperature. Look it up in a reference
table and then subtract it from the total
pressure.

24. Extension of Dalton’s Law
•Since PV = nRT then
•P
tot
= n
tot
RT & for each component, P
i
= n
i
RT
P
i = n
iRT/V
Which reduces to P
i / P
tot = n
i / n
tot
P
tot n
totRT/V
V V

25. Mole Fraction
•n
i
/ n
tot
is called the mole fraction = 
I
•P
i = (n
i / n
tot)P
tot

26. Effusion
vacuumgas

27. Effusion
•Process where molecules of a gas confined in
a container randomly pass through a tiny
opening in the container.
•Rates of effusion can be used to determine
the molar mass of a gas.
•Kinetic energy of molecules in a gas depends
only on temperature and equals ½ mv
2
.

28. Effusion
Two gases at the same T have the
same average KE:
½ M
1V
1
2
= ½ M
2V
2
2
V
1
2
= M
2
V
2
2
M
1
Note: This
V is the
velocity of
the
molecules,
NOT the
volume of
the
container.

29. Effusion
Two gases at the same T have the
same average KE:
½ M
1V
1
2
= ½ M
2V
2
2

30. Graham’s Law of Effusion
•The rates of effusion of gases at the same
pressure and temperature are inversely
proportional to the square roots of their
molar masses.
•The heavier gases move more slowly.

31. Effusion
•Recall: the faster you are going, the less time
it takes you to get somewhere.

32. Diffusion
•Gradual mixing of two or more molecules due
to their spontaneous, random motion.
Gases are in constant, random
motion and tend to move from
regions of higher concentration
to regions of lower concentration.

33. Diffusion

34. Molecular speed and Temperature

35. Molecular speed and mass