# What is Pressure? P= Force/area Pressure units are N/m 2 or Pascal(Pa)

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What is Pressure? P= Force/area Pressure units are N/m 2 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 14.7lbs.

How does that pressure feel? The behavior of a gas depends very strongly on the temperature and the pressure of the gas. STP. 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 = 0°C or 273K  Standard Pressure = 1atm or 760mmHg or 101.3kPa (Units depend upon the method of measure)

Why is a mmHg called a Torr? inability  Evangelista Torricelli was born in Italy in 1608. Due to the great Italian, Galileo’s 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.

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 1 st 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!

Sucks So if nothing Sucks 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 Pressure High atmospheric Pressure Mouth must make a seal

Pressure pushes 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 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 P total = P 1 +P 2 +P 3 +…

Dalton’s Law of Partial Pressure P T =P 1 +P 2 +P 3 +… John Dalton 1766-1844

Three of the primary components of air are CO 2, N 2, and O 2. In a sample containing a mixture of these gases at exactly 760 mmHg, the partial pressures of CO 2 and N 2 are given as P CO 2 = 0.285mmHg P N 2 = 593.525mmHg. What is the partial pressure of O 2 ? Three of the primary components of air are CO 2, N 2, and O 2. In a sample containing a mixture of these gases at exactly 760 mmHg, the partial pressures of CO 2 and N 2 are given as P CO 2 = 0.285mmHg P N 2 = 593.525mmHg. What is the partial pressure of O 2 ? Simple Dalton’s Law Calculation

P T = P CO2 + P N2 + P O2 Simple Dalton’s Law Calculation 760 mmHg =.285 mmHg + 593.525 mmHg + P O2 P O2 = 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 Pressure

P tot = P atmospheric pressure = P gas + P H 2 O Atmospheric Pressure Atmospheric Pressure —The water’s vapor pressure can be determined from a list and subtract- ed from the atmospheric pressure

WATER VAPOR PRESSURES Temp (°C)Vapor pressure (kPa) 10.65176 5.87260 101.2281 151.7056 202.3388 253.1691 304.2455 355.6267 407.3814 459.5898 5012.344

WATER VAPOR PRESSURES Temp (°C)Vapor pressure (kPa) 5515.752 60.19.932 6525.022 7031.176 7538.563 8047.373 8557.815 9070.117 9584.529 100101.32 105120.79

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. Dalton’s Law Calculation P H2O at 20.0°C= 2.3388 kPa We need to convert to mmHg.

P T = P H2O + P O2 Dalton’s Law Calculation 730.0 mmHg = 17.5468 + P O2 P O2 = 712.5 mmHg P H2O = 17.5468 mmHg 2.3388 kPa760 mmHg = 17.5468 mmHg 1 101.3 kPa

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