1. Pressure
Unit 5
Chapters 10 & 11

2. 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.

3. 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)

4. 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.

5. Weight of the World III
The total weight of atmosphere on top of an object:
Calculate surface area (use in2)
Multiply by 14.7 psi
How much pressure on the desk?
24.25” x 18.25” = in2
442.6 in2 x 14.7 psi = about 6500 lbs

6. 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.

7. Measuring It
Gas Pressure can be measured using a Manometer.

8. In calculations, you have
Equal to Each Other
Additional units for measuring air pressure
psi
1 atm (exact)
760 mmHg (exact)
760 torr (exact)
inHg
bar (exact)
101,325 Pa (exact)
kPa (exact)
These values are
all equal to each other!
In calculations, you have
to use the correct # of
Sig Figs for
psi & inHg.

9. Conversions Use equalities to interconvert
What is the pressure in mmHg if the news gives it as inHg?
29.85 inHg  inHg * 760 mmHg
4 sig figs
758.2 mmHg

10. 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!!!

11. 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 = = 90.8 atm
90.8 atm  1 atm * 14.7 psi = 1,334.76
1,330 psi (3 sig figs)

12. 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

13. Gas Collection Density higher than air Density lower than air
Low solubility in water

14. 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.

15. Vapor Pressure

16. 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 mmHg, what is the partial pressure of the gas?
764.7 – 17.3 = mmHg

17. Phasing it Out
Melting & Boiling points are directly affected by two things:
Temperature
Pressure
The phase boundaries vary depending on those two factors.

18. Phase Diagrams Triple Point Critical Point Sublimation / Deposition
Boiling / Condensing
0°C

19. 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

20. Moles at STP Original STP was 0°C & 1 atm
1 mole of any gas occupies 22.4 L 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)

21. Check More Grey Matter What vol does 50.0 g of O2 require at STP?
n = m / Mw
= 50.0 g / (16.00 * 2)
= = 1.56 moles O2
V = moles O2 * 22.4 L/mol
= 35
= 35.0 L O2 (3 sig figs)