Pressure

Unit 10Chapter 13

The Weight of the World

The atmosphere is 78% N2, 21% O2, 1% Ar, and < 1% other gases.

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

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

Weight of the World II

A 1 in2 column of air weighs approximately 14.7 lbs.

This is defined as 1 atmosphere of pressure

1 atm = 14.7 psi (pounds per square inch = lb/in2)

Forcing the Issue

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

The formula is: P = Force Area (P = F/A) When dealing with the

atmosphere, Pressure can be thought of as the

weight of the atmosphere on top of an object.

Weight of the World III

The total weight of atmosphere on top of an object: Since P = F/A, then F = P*A

Calculate surface area (use in2) Multiply by 14.7 psi What force is being exerted on the

desk? L = 24.25”, W = 18.25” 24.25” x 18.25” = 442.6 in2

442.6 in2 x 14.7 psi = about 6500 lbs

When is Length =To Pressure?!? 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 (enclosed).

Or a Barometer (open)

Equal to Each Other

Additional units for measuring air pressure 14.6959488 psi 1 atm (exact) 760 mmHg (exact) 760 torr (exact) 29.92125984 inHg 1.01325 bar (exact) 101,325 Pa (exact) 101.325 kPa (exact)

These values are all equal to each other!

In calculations, you haveto use the correct # of

Sig Figs for psi & inHg.

Conversions

Use equalities to interconvert What is the pressure in mmHg if

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

mmHg 758.2219251 4 sig figs 758.2 mmHg

Partially Acceptable

The Law of Partial Pressures

Declared by John Dalton (remember him?)

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

PT = P1 + P2 + P3 + …Pneumatic Chemist #

1!!!

Check some Grey Matter

Venus’ atmosphere consists of CO2 & N2.

What is the total pressure on Venus in psi? CO2 partial pressure = 87.6 atm N2 partial pressure = 3.2 atm

PT = 87.6 + 3.2 = 90.8 atm 90.8 atm 1 atm * 14.7 psi =

1,334.76 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:

1 T

1 T

P P

n n

Air Vs. Can Pour a little water in an empty soda

can. Place on hot plate (on high) Fill a large beaker with water and set

to the side of the hot plate. Take measurements of other can then

add water and put on hot plate, also. Calculate the surface area of the cans. When soda can boils, wait ~30

seconds. Quickly turn can into top of filled

beaker. Observe.

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 / Condensing

0°C

Standard Stuff

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

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

Moles at STP

Original STP was 0°C & 1 atm 1 mole of any gas (including

diatomics) occupies 22.4 Liters at STP

We can measure amount of gas without weighing it.

First, calculate the number of moles, and then multiply by 22.4 L/mol to get volume (in Liters)

For this class, use these values for

STP!

Check More Grey Matter

What vol does 50.0 g of O2 require at STP?

n = m / Mw = 50.0 g / (16.00 * 2) = 1.5625 = 1.56 moles O2

V = 1.5625 moles O2 * 22.4 L/mol = 35 = 35.0 L O2 (3 sig figs)