Pressure Unit 5 Chapters 10 & 11

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: Calculate surface area (use in2) Multiply by 14.7 psi How much pressure on the desk? 24.25” x 18.25” = 442.6 in2 442.6 in2 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.

In calculations, you have 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 have to 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: 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 / 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 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)

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)