13.2 Using Gas Laws to Solve Problems by dandanhuanghuang

VIEWS: 3 PAGES: 23

• pg 1
```									13.2 Using Gas Laws to Solve
Problems

QuickTime™ and a
decompressor
are neede d to se e this picture.

http://www.phy.cuhk.edu.hk/contextual/heat/tep/trans/kinetic_theory.gif
Today…
We are going to learn about:
the ideal gas law
partial pressure
molar volume

http://chemwiki.ucdavis.edu/@api/deki/files/4262/=SLOWEDEFFUSION.gif
The Ideal Gas Law
• Boyle’s Law   V = k (at constant T and n)
P
• Charles’s Law V = bT (at constant P and n)
• Avogadro’s Law V = an (at constant T and P)

We can combine these equations to get
The Ideal Gas Law

R = the universal gas constant= 0.08206 L atm
mol K

Rearrange to get the Ideal Gas Law:
PV = nRT
PV = nRT

The ideal gas law involves all the important
characteristics of a gas:
P - Pressure (in atm)
V - Volume (in L)
n - number of moles
T - Temperature (in K)

http://chemwiki.ucdavis.edu/@api/deki/files/4260/=WOOORRRKKKKK.gif
PV = nRT
Knowing any three of these properties is
describe the
enough to completely an d a
QuickTime™
the gas.
conditions ofdecompressor
are need ed to see this p icture.
The fourth property can be derived.
Setbacks to Ideal Gas Law
A gas that obeys this equation is said to
behave ideally.
However, in actuality, no gas is truly ideal.
Ideal Gas- A hypothetical gas that exactly
obeys the ideal gas law.
Why do we use it?
Most gases closely obey the ideal gas law at
pressures of approximately 1 atm or lower,
when the temperature is approximately 0oC
or higher.

http://chemistry.boisestate.edu/people/richardbanks/inorganic/chemistry/gas_animation.gif
Using the Ideal Gas Law
A sample of hydrogen gas, H2, has a volume of
8.56 L at a temperature of 0oC and a pressure
of 1.5 atm. Calculate the number of moles of
H2 present in this gas sample.
Using the Ideal Gas Law
A 2.50 mol sample of nitrogen gas has a
volume of 5.50 L at a temperature of 27oC.
Calculate the pressure of the nitrogen gas.
Using the Ideal Gas Law
What volume is occupied by 0.250 mol of
carbon dioxide gas a 25oC and 371 torr?
Ideal Gas and Changing Conditions
Suppose we have a 0.240-mol sample of ammonia gas
at 25oC with a volume of 3.5 L at a pressure of 1.68
atm. The gas is compressed to a volume of 1.35 L at
25oC. Find the final pressure.
Ideal Gas and Changing Conditions
A sample of methane gas that has a volume of 3.8 L at
5oC is heated to 86oC at constant pressure. Calculate
its new volume.
Using the Ideal Gas Law
The idea behind these exercises is to use the
ideal gas law for all types of gas law
problems.
We don’t have to ask, “Is this a Boyle’s Law
problem or a Charles’s Law problem?”

http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/atoms_quantum/kinetic%20gas%20animation.gif
Dalton’s Law of Partial Pressures
We just learned about the different
properties of gases: pressure, volume,
moles and temperature.
But we’ve only been concerned about
non-mixtures of gases.
What happens if we mix different gases
together?

http://www.nhn.ou.edu/~jeffery/course/c_energy/energyl/lec004/gas_001_kinetics.gif
Dalton’s Law of Partial Pressures
Many important gases contain a mixture
of components.
Air = 78% N2, 21% O2, 0.93% Ar…
Gaseous mixtures show that each
QuickTime™ an d a
decompressor
are need ed to see independently of
component behaves this p icture .
the others.
In other words, a given amount of oxygen
exerts the same pressure in a 1.0-L
container whether it is alone or in the
presence of nitrogen or a different gas.

http://www.chemistryland.com/CHM107/AirWeBreathe/Comp/AirAtomsMolecules.jpg
Dalton’s Law of Partial Pressures
For a mixture of gases in a container, the total pressure
exerted is the sum of the partial pressures of the
gases present.
Ptotal = P1 + P2 + P3…

The partial pressure of a gas is the pressure that a gas
would exert if it were alone in the container.
Each gas is responsible for only a part of the pressure.
Dalton’s Law of Partial Pressures
Same container =
same volume and temperature

P1=n1RT/V    P2=n2RT/V                                P3=n3RT/V

Ptotal = n1RT/V + n2RT/V…
= (n1 + n2 …) (RT/V)
QuickTime™ and a
decompressor
are neede d to see this picture.

= ntotal (RT/V)

https://reich-chemistry.wikispaces.com/file/view/P3.JPG/34106723/P3.JPG
Practice with Partial Pressure
Mixtures of helium and oxygen are used in the “air”
tanks of underwater divers for deep dives. For a
particular dive, 12 L of O2 at 25oC and 1.0 atm and 46
L of He at 25oC and 1.0 atm were pumped into a 5.0-L
tank. Calculate the partial pressure of each gas and
the total pressure in the tank at 25oC.
Practice with Partial Pressure
A 2.0-L flask contains a mixture of nitrogen gas and
oxygen gas at 25oC. The total pressure of the
gaseous mixture is 0.91 atm, and the mixture is
known to contain 0.050 mol of N2. Calculate the
partial pressure of oxygen and the moles of
oxygen present.
Gas Stoichiometry
So far, we have seen how useful the ideal
gas equation is.
It is useful for finding the number of moles of
gas involved.
QuickTime™ and a

This fact makes it possible to do
decompressor
are neede d to see this picture.

stoichiometric calculations for reactions
involving gas.

http://staff.um.edu.mt/jgri1/teaching/che2372/notes/10/10_16.gif
Practice
Calculate the volume of oxygen gas produced at 1.00
atm and 25oC by the complete decomposition of 10.5
g of potassium chlorate. The balanced equation for
the reaction is:
2KClO3  2KCl + 3O2
Molar Volume
It is useful to define the volume occupied by 1 mol
of a gas under certain conditions.
1 mol of an ideal gas at 0oC (273 K) and 1 atm,
has a volume of 22.4aL.
QuickTime™ and
decompressor
Standard Temperature and Pressure (STP): 0oC
are neede d to se e this picture.
and 1 atm
Molar Volume of an ideal gas is 22.4 L at STP.
22.4 L contains 1 mol of an ideal gas at STP.

http://blog.makezine.com/archive/2009/11/18/molar_beach_ball.jpg
STP
A sample of nitrogen gas has a volume of
1.75 L at STP. How many moles of N2
are present?

```
To top