What is Pressure?

1
What is Pressure?

• P Force/area
• Pressure units are N/m2 or Pascal(Pa)

2
How is pressure created by gas molecules?

• A gas is made up of various molecules. These
  molecules move around and collide with each other
  and the walls of their container. Gas molecules
  exert pressure on any surface with which they
  collide.

3
What is atmospheric Pressure?

• Atmospheric pressure is the pressure created by
  the mass of the gas molecules in the air being
  pulled to the surface of the Earth.
• Atmospheric pressure is a
  result of the fact that air has mass and is
  attracted by gravity producing a force.
• Air pressure can vary daily.

One Square inch of atmosphere weighs 14.7lbs.
4
How does that pressure feel?

• The behavior of a gas depends very strongly on
  the temperature and the pressure of the gas.
• To make it easier to discuss the behavior of a
  gas, it is convenient to designate a set of
  standard conditions, called STP.
• Standard Temp and Standard Pressure
• Standard Temperature 0C or 273K
• Standard Pressure 1atm or 760mmHg or 101.3kPa
• (Units depend upon the method of measure)

5
Why is a mmHg called a Torr?

• ? Evangelista Torricelli was born in Italy in
  1608. Due to the great Italian, Galileos
  suggestion, Torricelli investigated the
  phenomenon of the inability of water pumps to
  raise water more than 33 feet.
• The theory was that nature abhorred a vacuum
  and when a pump created a vacuum above a water
  surface the water rushed in to relieve the empty
  space.
• Torricelli inverted a 4-foot tube filled with
  mercury in a bowl of mercury and found that the
  level only dropped to about 30 inches above the
  bowl.

6
Torricelli invented the barometer

• Torricelli had demonstrated that air had weight
  and exerted pressure.
• He also noted that the level of mercury in the
  tube changed from day to day!
• The 1st weather man

7
What does the invention of the Barometer mean?

• That .
• NOTHING SUCKS!
• HUH?...

The Vacuum Sweeper your mother keeps telling you
to use doesnt SUCK!

• You do not SUCK on a straw.

YOU dont even SUCK face when you Kiss!
8
So if nothing Sucks what happens?

• Everything ..
• PUSHES!!

9
Lets look at how a straw works.

• Gas molecules always move from high to low areas
  of pressure.
• As you Suck on the straw you create a low
  pressure in your mouth. The High atmospheric
  pressure then PUSHES the fluid up the straw into
  the mouth.

Low Pressure
High atmospheric Pressure
Mouth must make a seal
10
Pressure

• Pressure always pushes from high pressure to low
  pressure.
• We measure the amount of push using either an
  open or closed manometer
• A closed manometer is the same as Torricellis
  barometer.

11
One atmosphere 760 mm of Hg 101 kPa So 1.00
mm of Hg 0.133 kPA
12
Daltons Law

• The pressure exerted by each gas in an unreactive
  mixture is independent of the pressure exerted by
  other gases.
• The pressure of each gas in a mixture is called
  the partial pressure of that gas.
• Daltons law of partial pressures states that the
  total pressure of a gas mixture is the sum of the
  partial pressures of the component gases.
• The Formula
• Ptotal P1P2P3

13
Daltons Law of Partial Pressure
John Dalton 1766-1844
PTP1P2P3
14
Simple Daltons Law Calculation
Three of the primary components of air are CO2,
N2, and O2. In a sample containing a mixture of
these gases at exactly 760 mmHg, the partial
pressures of CO2 and N2 are given as PCO2
0.285mmHg PN2 593.525mmHg. What is the
partial pressure of O2?
15
Simple Daltons Law Calculation
PT PCO2 PN2 PO2
760mmHg .285mmHg 593.525mmHg PO2
PO2 167mmHg
16
Daltons Law of Partial Pressure

• Partial Pressures are also important when a gas
  is collected over water.
• Any time a gas is collected through water the gas
  is contaminated with water vapor.
• You can determine the pressure of the dry gas by
  subtracting out the water vapor

17
Atmospheric Pressure
Ptot Patmospheric pressure Pgas PH2O

• The waters vapor pressure can be determined from
  a list and subtract-ed from the atmospheric
  pressure

18
WATER VAPOR PRESSURES WATER VAPOR PRESSURES
Temp (C) Vapor pressure (kPa)
1 0.65176
5 .87260
10 1.2281
15 1.7056
20 2.3388
25 3.1691
30 4.2455
35 5.6267
40 7.3814
45 9.5898
50 12.344
19
WATER VAPOR PRESSURES WATER VAPOR PRESSURES
Temp (C) Vapor pressure (kPa)
55 15.752
60 .19.932
65 25.022
70 31.176
75 38.563
80 47.373
85 57.815
90 70.117
95 84.529
100 101.32
105 120.79
20
Daltons Law Calculation
Determine the partial pressure of oxygen
collected by water displacement if the water
temperature is 20.0C and the total pressure of
the gases in the collection bottle is 730.0
mmHg.
PH2O at 20.0C 2.3388 kPa
We need to convert to mmHg.
21
Daltons Law Calculation
2.3388 kPa 760 mmHg 17.5468 mmHg
1 101.3 kPa 17.5468 mmHg
PH2O 17.5468 mmHg
PT PH2O PO2
730.0 mmHg 17.5468 PO2
PO2 712.5 mmHg