The Gas Laws
Of the three common states of matter, the
gases might seem to have the most unusual, yet consistent, properties. You
have already learned, for example that ALL gases occupy approximately the same volume per mole
under conditions of STP (standard temperature and pressure - that is 0º
celsius or 273K, and 760mm pressure (1 atmosphere or 101.3 kilopascals).
That standard volume is 22.4 liters for all gases. Thus, one mole of
oxygen, for example, which has a mass of 32g, occupies a volume of 22.4L at STP.
22.4L of hydrogen gas would have a mass of 2g. A mole of carbon dioxide
(44g) would likewise occupy 22.4L, etc.
The
problem of course comes in arriving at a volume for a given number of moles or
mass of gas if conditions are not at STP...
Boyle'
Law <-- Take this link to a
page of good discussion about Boyle's law.
|
As you see, Robert Boyle gave us a way to
calculate the volume of a gas when the pressure on a closed, but
flexible system is changed. His formula specifically states that
the volume of a gas, under conditions of stable temperature and amount
of gas, is inversely proportional to pressure. Thus, if the
pressure is doubled, the volume of the gas will be cut in half,
etc. The formula for this law is:
P1V1 = P2V2
whereas, P1 is the original or
starting pressure, P2 is the final pressure, V1 is
the starting volume and V2 is the final volume. We can
use this formula to solve for either one of the four variables if the
other three are known, for example:
V2 = P1V1 / P2
Take care in using Boyle's Law that you
remember that the temperature or amount of gas (moles or mass) cannot
change at the same time!
|
Robert
Boyle
|
Charles'
Law <--- Go to this site for a good
explanation of Charles' Law.
|
Jacques
Charles |
J. Charles discovered a way for us to
understand the relationship between the volume of a gas and its temp-
erature. After a series of experiments, he concluded that the volume of
a gas is DIRECTLY proportional to its absolute temperature. (V1
/ T1 = V2 / T2) . This works ONLY
if any temperature given is first converted to absolute (Kelvin)
temperature and assuming that the amount (moles or mass) of gas and the
pressure are held constant. It has also been noticed that the
pressure changes if the volume is held constant and
the temperature is changed. Click
here < for a really neat site which will
actually allow you to see what happens to the pressure in a closed
container (volume held constant) when the temperature is changed.
Note that the temperature must be in Kelvin when working Charles' Law
problems! |
Gay-Lussac's
Law <---- Click on this link for a more thorough page of explanation.
The
Combined Gas Law - This link takes you to an explanation of the derivation
of the law.
The
Ideal Gas Law - This
link gives you an explanation of the derivation of this law.
| The following links will help in your understanding of
this law:
http://jersey.uoregon.edu/vlab/Piston/
http://jersey.uoregon.edu/vlab/Piston/
http://www.phy.ntnu.edu.tw/java/idealGas/idealGas.html
http://dbhs.wvusd.k12.ca.us/GasLaw/WS-Ideal.html
|
The big story here is PV=nRT, where P=pressure, V=volume,
n= moles of gas, R=the gas constant, and T=the temperature. Note
that R may be expressed with pressure in atmospheres or in Kpa or
another way. Be sure to be consistent with your units of
measurement when using this formula. V is measured in liters (L),
P in atm. or mmHg or Kpa, n in moles, and T in Kelvins. The neat
thing about using this formula is that since all gases have a definite
density (from mass and volume) at STP, the formula can be coordinated
with the density formula to actually determine other things such as the
molecular weight, etc. |
Dalton's
Law <----- This web site will assist
you in understanding this law.
|
|
Dalton's Law of Partial Pressures is pretty
simple and logical...
It simply states that the total pressure exerted by a
mixture of gases is equal to all of the individual (partial) pressures
of the gases in the mixture added together. So the pressure of the
first gas plus the pressure of the second gas, etc. etc. plus the
pressure of the last gas = the total pressure of the mixture of
gases. Dalton's Law can be useful in coordination with any or all
of the other gas laws to calculate numerous significant engineering
problems. |
The following links will be useful for further study and for
finding practice problems related to this law:
http://www.fordhamprep.com/gcurran/dlawcalc.htm
http://dbhs.wvusd.k12.ca.us/GasLaw/Gas-Dalton.html
http://www.chm.davidson.edu/ChemistryApplets/GasLaws/DaltonsLaw.html
http://www.fordhamprep.com/gcurran/dlawcalc.htm
Graham's Law of Effusion of Gases
|
Graham's Law deals with the effusion of gases.
This is not to be confused with diffusion which declares that molecules
will move from a place of higher concentration to a place of lower
concentration. The principle of effusion depends upon a movement
or diffusion of gases but it relates to the rate of travel of a gas
through a predefined pathway and how the rate depends upon the molecular
mass of the gas. Specifically, the law states that the relative
rates of effusion of two gases through a tube or pathway of equal size
is inversely proportional to the square root of the molecular masses of
the gases. The formula is: 
Some other interesting sites related to this law are:
http://scienceworld.wolfram.com/physics/GrahamsLawofEffusion.html
Graham's
Law Experiment
|
Thomas
Graham |
Standards of Learning Practice Problems and Questions:
(multiple choice)
1. One of the main assumptions of the kinetic
molecular theory of gases is that particles of an ideal gas a. must be single
atoms instead of molecules. b. are in constant motion. c. must be maintained at
very high pressures. d. must be highly chemically reactive.
2. The average kinetic energy of a sample of gas
molecules is a. increased as the temperature is decreased b.
increased as the temperature is increased. c. unaffected by temperature
changes d. always equal to zero.
3. If the pressure on a confined gas is doubled, then
the volume of the gas a. increases four times b.
decreases by one-fourth c. is doubled d. is halved.
4. Which volume will be occupied by a gas containing
6.02 X 1023 atoms at STP? a. 1.0L b.
11.2L c. 22.4L d. 44.8L
5. A sample of oxygen gas is collected over water at
22º
C and 98.67kPa pressure. If the pressure of the water (vapor) is 2.67, the
partial pressure of the oxygen is a. 93.33kPa b.
96.00kPa c. 98.66kPa d. 101.33kPa
©2002 Ron Swindall
permission required to copy!