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Pressure
The force per unit area of a surface.Units: N/cm2
N, Newton: SI unit of forceAs area of contact changes, force changes500 N = 1.7 N 500 N = 83.3 N300 cm2 cm2 6.0 cm2 cm2
Christina,
Will you go to the prom with me?
Steve
Barometer
• Used to measure the pressure of gases.
barometerplanet.com
Units of Pressure for Gases
• Millimeters of mercury (mm Hg)• Torricelli or 1 torr = 1mm Hg • 760 mm Hg = 1 atmosphere at sea level
when temp is 0oC• Pascal = pressure exerted by a force of one
newton acting on an area of one square meter. Pa = N/m2
• 1.013 x 105 kPa = 1 atmosphere= 10.1N/cm2
Pressure conversion sample:
Express 0.725 atm in a) mm Hg and b) kilopascals (kPa)
A) 0.725 atm x 760 mm Hg = 551 mm Hg 1 atm
B) 0.725 atm x 101.325 kPa = 73.5 kPa 1 atm
Standard Conditions (STP)
Standard Temperature is 0oC or 273K
Standard Pressure is 1 atm or 760 mm of Hg
You must have done by next meeting:
A list of the units of pressure (see page 364) andA list of the gas laws:Dalton’s Law of Partial Pressures: PT = P1+P2+…
Boyle’s: P1V1 = P2V2 T constant
Charles’s: V1 = V2 P constant
T1 T2
Gay-Lussac’s: P1 = P2 V constant
T1 T2
Combined Gas: P1V1 = P2V2
T1 T2
Ideal Gas: PV = nRT
Dalton’s Law of Partial Pressures
The total pressure of a gas mixture is the sum of the partial pressures of the component gases.
Gas collected through water picks up water vapor, so allow inside and outside water levels in a gas collection device to stabilize and:
Patm = Pgas + PH20
Dalton sample problem:
Oxygen gas is collected by water displacement. The barometric pressure and the temperature during the experiment are 731.0 torr and 25.0oC. What was the partial pressure of the oxygen collected?
PT = Patm = 731.0 torr
PH20 = 23.8 torr (see vapor pressure of water at 25.0 oC from table in handout or book – Table A-8)
PT = Patm = 731.0 torr
PH20 = 23.8 torr (see vapor pressure of water at 25.0 oC from table in handout or book – Table A-8)
Patm = PO2 + PH20
So PO2 = Patm – PH20
PO2 = 731.0 torr – 23.8 torr = 707.2 torr
Boyle’s Law – at constant temperature, volume of a fixed gas varies inversely with the pressure.
If 100.0 mL of a gas, originally at 760 torr, is compressed to a pressure of 800 torr, at a constant temperature, what would be its final volume?
P1V1 = P2V2 --> V2 = P1V1
P2
V2 = 100mL(760 torr) = 95.0 mL
800 torr
Charles’ Law – the volume of a fixed mass of gas at constant pressure varies directly with the
Kelvin temperature.
A sample of neon gas has a volume of 752 mL at 25.0oC. What will the volume at 100.0oC be if pressure is constant?
V1 = V2 --> V2 = V1T2
T1 T2 T1
V2 = 752 mL (100.0oC) = 300.8 mL
25.0oC
Gay-Lussac’s Law – the pressure of a fixed mass of gas at constant volume varies directly with
the Kelvin temperature.
At 122oC the pressure of a sample of nitrogen gas is 1.07 atm. What will the pressure be at 205oC, assuming constant volume?
P1/T1 = P2/T2 --> P2 = P1T2
T1
P2 = 1.07 atm(205&273) = 1.29 atm
122+273
Gay-Lussac’s Law of combining volumes
At constant temperature and pressure, the volumes of gaseous reactants and products can be expressed as ratios of small whole numbers.
H2 + Cl2 --> 2HCl
1L 1L 2L H:Cl:HCl = 1:1:2
Formulas must be written correctly and chemical equation balanced.
Combined Gas Law – expresses the relationship between pressure, volume, and temperature of
a fixed amount of a gas. PV = k T
P1V1 = P2V2 --> To find V2: V2 = P1V1T2
T1 T2 P2T1
Problem: The volume of a gas is 27.5mL at 22.0oC and 0.974 atm. What will be the volume at 15.OoC and 0.993 atm?Temps to K: 22+273 = 295K and 15+273=288K
The volume of a gas is 27.5mL at 22.0oC and 0.974 atm. What will be the volume at 15.OoC and 0.993 atm?
V2 = P1V1T2
P2T1
V2 = 0.974atm(27.5ml)(288K)
0.993atm(295K)V2 = 26.3 mL
Hint: in solving gas law problems, use the combined gas law and quantities that don’t change will cancel out.
V1 = 27.5mlT1 = 295KP1 = 0.974atmV2 = ?T2 = 288KP2 = 0.993atm
Avogadro’s Law – equal volumes of gases at the same temperature and pressure contain equal
numbers of molecules.Ratios apply here also.2H2 + O2 --> 2H2O
2molecules 1molecule 2molecules2mol 1mol 2mol2volumes 1volume 2volumesStandard molar volume of a gas is the volume
occupied by one mole of a gas at STP.Standard molar volume = 22.4 L/mol
Steve,
Yes, I will go to the prom with you.
Christina
At STP, what is the volume of 7.08 mol of nitrogen gas?
7.08 mol (22.4L) = 158 L 1 mol
A sample of gas occupies 11.9 L at STP. How many moles of the gas are present?
11.9L (1 mol) = 0.531 mol 22.4L
Assuming all volume measurements are made at the same temperature and pressure, what volume of
hydrogen gas is needed to react completely with 4.55 L of oxygen gas to produce water vapor?
Write and balance the equation.
Label known and unknown.
Do unit analysis.
Gas Stoichiometry – dealing with proportional relationships between reactants and products in
a chemical reaction.2CO2(g) + O2(g) --> 2CO2(g)
2molecules 1 molecule 2 molecules2 mol 1 mol 2 mol2 volumes 1 volume 2 volumes
Assuming all volume measurements are made at the same temperature and pressure, what volume of hydrogen gas is needed to react completely with 4.55 L of oxygen gas to produce water vapor?
Write the correct chemical reaction first:
2H2 + O2 --> 2H2O
Indicate known and unknown:Solve:
Ideal Gas Law – the mathematical relationship among pressure, volume, temperature, and the
number of moles of a gas.PV = nRT --> R = PV R is the ideal gas
nT constant.At STP, R = 1 atm(22.414 L) = 0.0821 L atm
1 mol(273.15K) mol K
R is the ideal gas constant.
What pressure, in atmospheres, is exerted by 0.325 mol of hydrogen gas in a 4.08 L container
at 35oC?PV = nRT --> P = nRT VT = 35 + 273 = 308KP = 0.325 mol 0.0821L atm) (308K) 4.08 L mol KP = 2.01 atm
Gases spread out in a container – diffusion
Gases can randomly pass through a tiny opening in a container (leak out) - effusion