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Gases balloonfestivals.com treehuggerusa.com treehugger.com

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Pressure The force per unit area of a surface. Units: N/cm 2 N, Newton: SI unit of force As area of contact changes, force changes 500 N = 1.7 N 500 N = 83.3 N 300 cm 2 cm 2 6.0 cm 2 cm 2

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Christina, Will you go to the prom with me? Steve

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Barometer Used to measure the pressure of gases. barometerplanet.com

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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 0 o C Pascal = pressure exerted by a force of one newton acting on an area of one square meter. Pa = N/m 2 1.013 x 10 5 kPa = 1 atmosphere= 10.1N/cm 2

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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

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Standard Conditions (STP) Standard Temperature is 0 o C or 273K Standard Pressure is 1 atm or 760 mm of Hg

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You must have done by next meeting: A list of the units of pressure (see page 364) and A list of the gas laws: Daltons Law of Partial Pressures: P T = P 1 +P 2 +… Boyles: P 1 V 1 = P 2 V 2 T constant Charless: V 1 = V 2 P constant T 1 T 2 Gay-Lussacs: P 1 = P 2 V constant T 1 T 2 Combined Gas: P 1 V 1 = P 2 V 2 T 1 T 2 Ideal Gas: PV = nRT

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Daltons 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: P atm = P gas + P H20

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Dalton sample problem: Oxygen gas is collected by water displacement. The barometric pressure and the temperature during the experiment are 731.0 torr and 25.0 o C. What was the partial pressure of the oxygen collected? P T = P atm = 731.0 torr P H20 = 23.8 torr (see vapor pressure of water at 25.0 o C from table in handout or book – Table A-8)

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P T = P atm = 731.0 torr P H20 = 23.8 torr (see vapor pressure of water at 25.0 o C from table in handout or book – Table A-8) P atm = P O 2 + P H20 So P O2 = P atm – P H20 P O2 = 731.0 torr – 23.8 torr = 707.2 torr

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Boyles 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? P 1 V 1 = P 2 V 2 --> V 2 = P 1 V 1 P2P2 V 2 = 100mL(760 torr) = 95.0 mL 800 torr

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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 25.0 o C. What will the volume at 100.0 o C be if pressure is constant? V 1 = V 2 --> V 2 = V 1 T 2 T 1 T 2 T 1 V 2 = 752 mL (100.0 o C) = 300.8 mL 25.0 o C

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Gay-Lussacs Law – the pressure of a fixed mass of gas at constant volume varies directly with the Kelvin temperature. At 122 o C the pressure of a sample of nitrogen gas is 1.07 atm. What will the pressure be at 205 o C, assuming constant volume? P 1 /T 1 = P 2 /T 2 --> P 2 = P 1 T 2 T 1 P 2 = 1.07 atm(205&273) = 1.29 atm 122+273

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Gay-Lussacs 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. H 2 + Cl 2 --> 2HCl 1L1L 2L H:Cl:HCl = 1:1:2 Formulas must be written correctly and chemical equation balanced.

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Combined Gas Law – expresses the relationship between pressure, volume, and temperature of a fixed amount of a gas. PV = k T P 1 V 1 = P 2 V 2 --> To find V 2 : V 2 = P 1 V 1 T 2 T 1 T 2 P 2 T 1 Problem: The volume of a gas is 27.5mL at 22.0 o C and 0.974 atm. What will be the volume at 15.O o C and 0.993 atm? Temps to K: 22+273 = 295K and 15+273=288K

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The volume of a gas is 27.5mL at 22.0 o C and 0.974 atm. What will be the volume at 15.O o C and 0.993 atm? V 2 = P 1 V 1 T 2 P 2 T 1 V 2 = 0.974atm(27.5ml)(288K) 0.993atm(295K) V 2 = 26.3 mL Hint: in solving gas law problems, use the combined gas law and quantities that dont change will cancel out. V 1 = 27.5ml T 1 = 295K P 1 = 0.974atm V 2 = ? T 2 = 288K P 2 = 0.993atm

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Avogadros Law – equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. Ratios apply here also. 2H 2 + O 2 --> 2H 2 O 2molecules1molecule2molecules 2mol1mol2mol 2volumes1volume2volumes Standard molar volume of a gas is the volume occupied by one mole of a gas at STP. Standard molar volume = 22.4 L/mol

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Steve, Yes, I will go to the prom with you. Christina

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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

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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.

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Gas Stoichiometry – dealing with proportional relationships between reactants and products in a chemical reaction. 2CO 2 (g) + O 2 (g) --> 2CO 2 (g) 2molecules1 molecule2 molecules 2 mol1 mol2 mol 2 volumes1 volume2 volumes

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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: 2H 2 + O 2 --> 2H 2 O Indicate known and unknown: Solve:

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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.

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What pressure, in atmospheres, is exerted by 0.325 mol of hydrogen gas in a 4.08 L container at 35 o C? PV = nRT --> P = nRT V T = 35 + 273 = 308K P = 0.325 mol 0.0821L atm) (308K) 4.08 L mol K P = 2.01 atm

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Gases spread out in a container – diffusion Gases can randomly pass through a tiny opening in a container (leak out) - effusion

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Gases. Define pressure, give units of pressure, and describe how pressure is measured. State the standard conditions of temperature and pressure and.

Gases. Define pressure, give units of pressure, and describe how pressure is measured. State the standard conditions of temperature and pressure and.

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