PressurePressure

Unit 5Unit 5

Chapters 10 & 11Chapters 10 & 11

The Weight of the WorldThe Weight of the World

The atmosphere is 78% NThe atmosphere is 78% N22, 21% O, 21% O22, , 1% Ar, and < 1% other gases. 1% Ar, and < 1% other gases.

99.9% of the Earth’s Atmosphere can 99.9% of the Earth’s Atmosphere can be found in the Troposphere (0 to 11 be found in the Troposphere (0 to 11 km) and Stratosphere (11 to 50 km).km) and Stratosphere (11 to 50 km).

All that gas has a weight. That weight All that gas has a weight. That weight is pushing down on us all the time.is pushing down on us all the time.

Weight of the World IIWeight of the World II

A 1 inA 1 in22 column of air column of air weighs weighs approximately 14.7approximately 14.7 lbs.lbs.

This is defined as 1 This is defined as 1 atmosphereatmosphere of of pressurepressure

1 atm = 14.7 psi 1 atm = 14.7 psi (pounds per square (pounds per square inch = lb/ininch = lb/in22))

Forcing the IssueForcing the Issue

Pressure is the amount of force being Pressure is the amount of force being exerted on a surface.exerted on a surface.

The formula is:The formula is: P = P = Force Force Area (P = F/A) Area (P = F/A) When dealing with the atmosphere, When dealing with the atmosphere, Pressure can be thought of as the Pressure can be thought of as the

weight of the atmosphere on top of weight of the atmosphere on top of an object.an object.

Weight of the World IIIWeight of the World III

The total weight of atmosphere on The total weight of atmosphere on top of an object:top of an object:

Calculate surface area (use inCalculate surface area (use in22)) Multiply by 14.7 psiMultiply by 14.7 psi How much pressure on the desk?How much pressure on the desk? 24.25” x 18.25” = 442.6 in24.25” x 18.25” = 442.6 in22

442.6 in442.6 in22 x 14.7 psi = about 6500 lbs x 14.7 psi = about 6500 lbs

Who Chose That?!? Nature abhors a

vacuum; however, mercury doesn’t mind it so much!

Air pushes down on a pool of mercury.

Height of 1” column of mercury ~760 mm.

Measuring It

Gas Pressure can be measured using a Manometer.

Equal to Each OtherEqual to Each Other

Additional units for measuring air Additional units for measuring air pressure pressure 14.6959488 psi14.6959488 psi 1 atm 1 atm (exact)(exact) 760 mmHg 760 mmHg (exact)(exact) 760 torr 760 torr (exact)(exact) 29.92125984 inHg29.92125984 inHg 1.013251.01325 bar bar (exact)(exact) 101,325 Pa 101,325 Pa (exact)(exact) 101.325 kPa101.325 kPa (exact) (exact)

These values are all equal to each other!

In calculations, you haveto use the correct # of

Sig Figs for psi & inHg.

ConversionsConversions

Use equalities to interconvertUse equalities to interconvert What is the pressure in mmHg if the What is the pressure in mmHg if the

news gives it as 29.85 inHg?news gives it as 29.85 inHg? 29.85 inHg 29.85 inHg 29.92 inHg * 760 29.92 inHg * 760

mmHgmmHg 758.2219251758.2219251 4 sig figs4 sig figs 758.2 mmHg758.2 mmHg

Partially AcceptablePartially Acceptable

The Law of Partial The Law of Partial PressuresPressures

Declared by John Dalton Declared by John Dalton (remember him?)(remember him?)

StatesStates Total Pressure is Total Pressure is the sum of the the sum of the individual pressures individual pressures added together.added together.

PPTT = = PP11 + P + P22 + P + P33 + … + … Pneumatic Chemist # Pneumatic Chemist # 1!!!1!!!

Check some Grey MatterCheck some Grey Matter

Venus’ atmosphere consists of COVenus’ atmosphere consists of CO22 & N & N22.. What is the total pressure on Venus in What is the total pressure on Venus in

psi?psi? COCO22 partial pressure = 87.6 atm partial pressure = 87.6 atm NN22 partial pressure = 3.2 atm partial pressure = 3.2 atm

PPTT = 87.6 + 3.2 = 90.8 atm = 87.6 + 3.2 = 90.8 atm 90.8 atm 90.8 atm 1 atm * 14.7 psi = 1,334.76 1 atm * 14.7 psi = 1,334.76 1,330 psi (3 sig figs)1,330 psi (3 sig figs)

PP & n

Assuming constant volume and temp in a mixture of gases, the partial pressure of a gas (P1) is proportional to the moles of the gas (n1), therefore:

P1/ PT= n1/nT

Gas Collection

Density higher than air Density lower than air Low solubility in water

Fun with Water! For gases collected over a liquid, the

partial pressure of the liquid must be accounted for.

Vapor is present because some of the molecules are able to escape the surface of the liquid.

The vapor pressure of the liquid is dependent on temperature.

Vapor Pressure

Attack of the Vapors

A sample of gas is collected over water at room temperature.

At that temperatures, water’s vapor pressure is 17.3 mmHg

If the total pressure is 764.7 mmHg, what is the partial pressure of the gas?

764.7 – 17.3 = 747.4 mmHg

Phasing it Out

Melting & Boiling points are directly affected by two things:

Temperature Pressure The phase boundaries vary

depending on those two factors.

Phase Diagrams

Triple Point Critical Point Sublimation / Deposition Boiling / Condensing0°

C

Standard StuffStandard Stuff

Since gases change a lot with Since gases change a lot with temperature & pressure, several temperature & pressure, several institutions define “standardinstitutions define “standard conditions”conditions”

Called “STP”Called “STP” IUPAC = 0°C & 1 barIUPAC = 0°C & 1 bar NIST = 20°C & 1 atmNIST = 20°C & 1 atm

Moles at STPMoles at STP

Original STP was 0°C & 1 atm Original STP was 0°C & 1 atm 1 mole of any gas occupies 22.4 L 1 mole of any gas occupies 22.4 L

at STPat STP We can measure amount of gas We can measure amount of gas

without weighing it.without weighing it. First, calculate the number of First, calculate the number of

moles, and then multiply by 22.4 moles, and then multiply by 22.4 L/mol to get volume (in Liters)L/mol to get volume (in Liters)

Check More Grey MatterCheck More Grey Matter

What vol does 50.0 g of OWhat vol does 50.0 g of O22 require at require at STP?STP?

n = m / Mwn = m / Mw = 50.0 g / (16.00 * 2)= 50.0 g / (16.00 * 2) = 1.5625 = 1.56 moles O= 1.5625 = 1.56 moles O22

V = 1.5625 moles OV = 1.5625 moles O22 * 22.4 L/mol * 22.4 L/mol = 35 = 35 = 35.0 L O= 35.0 L O22 (3 sig figs) (3 sig figs)