What is Pressure?

P= Force/area

Pressure units are N/m2 or Pascal(Pa)

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.

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

14.7lbs14.7lbs..

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.STP. Standard Temp and Standard

Pressure

Standard Temperature = 0°C or 273KStandard Pressure = 1atm or 760mmHg or

101.3kPa (Units depend upon the method of

measure)

Why is a mmHg called a Why is a mmHg called a TorrTorr??

Evangelista Torricelli was born in Italy in 1608. Due to the great Italian, Galileo’s suggestion, Torricelli investigated the phenomenon of the inabilityinability 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.

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…

What does the invention of the Barometer mean?

That ….

NOTHING SUCKS!

HUH?...You do not

SUCK on a straw.

The Vacuum Sweeper your mother keeps

telling you to use doesn’t SUCK!

YOU don’t even SUCK face when

you Kiss!

So if nothing SucksSucks what happens?Everything …..

PUSHES!!

Let’s 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

Press

ure

High atmospheric Pressure

Mouth must

make a se

al

Pressure

Pressure always pushes 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 Torricelli’s barometer.

One atmosphere = 760 mm of Hg = 101 kPa So 1.00 mm of Hg = 0.133 kPA

Dalton’s 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.

Dalton’s 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 = P1+P2+P3 +…

Dalton’s Law of Partial PressureDalton’s Law of Partial Pressure

PT=P1+P2+P3+…PT=P1+P2+P3+…

John DaltonJohn Dalton1766-18441766-1844

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?

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?

Simple Dalton’s Law Calculation

Simple Dalton’s Law Calculation

PT = PCO2 + PN2 + PO2PT = PCO2 + PN2 + PO2

Simple Dalton’s Law Calculation

Simple Dalton’s Law Calculation

760mmHg = .285mmHg + 593.525mmHg + PO2

760mmHg = .285mmHg + 593.525mmHg + PO2

PO2= 167mmHgPO2= 167mmHg

• 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

• 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

Dalton’s Law of Partial PressureDalton’s Law of Partial Pressure

Ptot = Patmospheric pressure = Pgas + PH2OPtot = Patmospheric pressure = Pgas + PH2O

AtmosphericPressure

AtmosphericPressure

—The water’s vapor pressure can be determined from a list and subtract-ed from the atmospheric pressure

—The water’s vapor pressure can be determined from a list and subtract-ed from the atmospheric pressure

WATER VAPOR PRESSURESWATER VAPOR PRESSURES

Temp (°C) Vapor pressure (kPa)

11 0.651760.65176

55 .87260.87260

1010 1.22811.2281

1515 1.70561.7056

2020 2.33882.3388

2525 3.16913.1691

3030 4.24554.2455

3535 5.62675.6267

4040 7.38147.3814

4545 9.58989.5898

5050 12.34412.344

WATER VAPOR PRESSURESWATER VAPOR PRESSURES

Temp (°C) Vapor pressure (kPa)

5555 15.75215.752

6060 .19.932.19.932

6565 25.02225.022

7070 31.17631.176

7575 38.56338.563

8080 47.37347.373

8585 57.81557.815

9090 70.11770.117

9595 84.52984.529

100100 101.32101.32

105105 120.79120.79

Determine the partial pressure of oxygen collected by water displacement if the water temperature is 20.0°C and the total pressure of the gases in the collection bottle is 730.0 mmHg.

Determine the partial pressure of oxygen collected by water displacement if the water temperature is 20.0°C and the total pressure of the gases in the collection bottle is 730.0 mmHg.

Dalton’s Law CalculationDalton’s Law Calculation

PH2O at 20.0°C= 2.3388 kPaPH2O at 20.0°C= 2.3388 kPa

We need to convert to mmHg.We need to convert to mmHg.

PT = PH2O + PO2PT = PH2O + PO2

Dalton’s Law CalculationDalton’s Law Calculation

730.0 mmHg = 17.5468 + PO2730.0 mmHg = 17.5468 + PO2

PO2= 712.5 mmHgPO2= 712.5 mmHg

PH2O = 17.5468 mmHgPH2O = 17.5468 mmHg

2.3388 kPa 760 mmHg= 17.5468

mmHg1 101.3 kPa