Gases

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Gases

• Chapter 10 BLB 11th

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Expectations

• Convert between different pressure units.
• Solve for any gas quantity from other given
  quantities.
• Include gases in stoichiometric calculations.
• Work with mole fractions and partial pressures.
• Calculate and compare the average velocities of
  gases.

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10.1 Characteristics of Gases

• Earths Atmosphere -
• dry air near sea level
• (from 18.1, p. 769)
• Other common gases (Table 10.1, p. 394)

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Characteristics of Gases, cont.

• 3 states of matter solid, liquid, gas
• Properties of gases
• Gases expand to fill a container.
• Gases are easily compressed.
• Gases mix readily and completely mix with other
  gases.
• Gases exert pressure on their surroundings.
• All gases behave similarly regardless of identity.

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Characteristics of Gases, cont.

• In order to fully describe a gas, four quantities
  are needed
• Pressure (P) in Pa (SI unit), atm, mm Hg, torr
• Volume (V) in L
• Temperature (T) in K
• Quantity of gas (n) in moles

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10.2 Pressure

• P F/A
• Measuring pressure
• Atmospheric pressure measured with a barometer.
  (p. 396)
• Gas pressure in a container measured with a
  manometer. (p. 397)
• Standard atmospheric pressure 1 atm
• 1 atm 760 torr 760 mm Hg 29.92 in Hg
  101,325 Pa 14.70 psi 1013.25 mBar

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Barometer
Manometer
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Pressure Conversions
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10.4 The Ideal-Gas Equation

• A gas which obeys this equation is said to behave
  ideally.

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Ideal-Gas Equation, cont.

• There are no ideal gases.
• A real gas approaches ideal behavior at low
  pressures and high temperatures, where molecules
  can behave (almost) independently. (section 10.9)
• Standard temperature pressure (STP)
• T 0C (273.15 K)
• P 1 atm

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Molar Volume

• Of an ideal gas at STP?
• Of a real gas?

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Calculate the following for a sample of Cl2 gas
with a volume of 9.22 L at 1124 torr and 24C.

• Mass of Cl2?
• Volume (in L) at STP?

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Cont. Cl2 gas with a volume of 9.22 L at 1124
torr and 24C.

• At what temperature will V 15.00 L if P 876
  torr?
• At what pressure will V 6.00 L if T 58C

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10.3 The Other Gas Laws
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Boyles Law
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Boyles Law for Real Gases
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Charless Law Absolute Zero
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Charless Law for Real Gases
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Avogadros Law Equal volumes of gases at the
same T and P contain equal numbers of molecules.
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A sample of gas occupies a volume of 1248 ft3 at
0.988 atm and 28.0C.

• Calculate the pressure if volume is decreased to
  978 ft3 at constant T.
• At what temperature in C is the volume 1435 ft3
  at constant P?

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10.28 Nitrogen and hydrogen gases react to form
ammonia gas N2(g) 3 H2(g) ? 2 NH3(g). At a
certain temperature and pressure, 1.2 L of N2
reacts with 3.6 L of H2. If all the N2 and H2
are consumed, what volume NH3 will be produced?
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10.5 Further Applications of PVnRT

• Gas Densities and Molar Mass (M)
• density (g/L)
• molar mass (g/mol)

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Calculate the density of SF6 at 455 torr and 32C.
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Calculate the molar mass of a vapor that has a
density of 6.345 g/L at 22C and 743 torr.
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• Gas Stoichiometry
• Use gas information to obtain moles.
• Otherwise, standard stoichiometry guidelines
  apply.

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10.54 Calcium hydride, CaH2, reacts with water to
form hydrogen gas. How many grams of CaH2 are
needed to generate 64.5 L of H2 gas if the
pressure of H2 is 814 torr at 32C? CaH2(s)
2 H2O(l) ? Ca(OH)2(aq) 2 H2(g)
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10.6 Gas Mixtures and Partial Pressures

• For a fixed-volume container
• pressures and moles are additive
• volume is fixed.
• Daltons Law of Partial Pressure
• Collecting gases over water

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Collection of a gas over water
Total gas collected product gas water
vapor Ptotal Pgas Pwater
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Gas Mixtures, cont.

• Mole fraction (X) ratio of moles of one
  substance to the total number of moles in mixture.

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Acetylene gas, C2H2(g), can be prepared by the
reaction of calcium carbide with water.
Calculate the volume of C2H2 that is collected
over water at 21C by reaction of 3.26 g of CaC2
if the total pressure of the gas is 748 torr?
The vapor pressure of water at 21C is 18.65
torr. CaC2(s) 2 H2O(l) ? Ca(OH)2(aq)
C2H2(g)
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A mixture of gases contains 0.75 mol N2, 0.30 mol
O2, and 0.15 mol CO2. If the total pressure of
the mixture is 1.56 atm, what is the partial
pressure of each component?
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A sample of 5.25 g of SO2(g) originally in a
4.00-L container at 26C is transferred to a
13.6-L container at 25C. A sample of 2.35 g of
N2(g) originally in a 3.18-L container at 20C is
transferred to this same 13.6-L container.(a)
What is the PSO2?(b) What is the PN2?
(c) What is the Ptotal?
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10.7 Kinetic-Molecular Theory

• Postulates (pp. 414)
• Particles are in constant, random motion.
• Particles are so small that the volume of each
  is negligible relative to the total volume.
• Particles are assumed to exert no forces on each
  other. (elastic collisions)
• Average kinetic energy is directly proportional
  to the absolute temperature. At a given T all
  gas particles have same average kinetic energy.

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Molecular Speeds for N2
Root-mean-square speed
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Molecular Speeds of Different Gases at 25C
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Molecular Speed

• Average kinetic energy is the same for any gas at
  a given T.
• ? But the mass of each gas is different.?
• Root-mean-square speed depends on mass and T.
• Lighter molecules move faster.
• Heavier molecules move more slowly.