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  • 2Importance of Gases

    Airbags fill with N2 gas in an accident.

    Gas is generated by the decomposition of sodium azide, NaN3.

    2 NaN3 ---> 2 Na + 3 N2


  • 4General Properties of Gases

    There is a lot of free space in a gas.

    Gases can be expanded infinitely.

    Gases fill containers uniformly and completely.

    Gases diffuse and mix rapidly.

  • 5Properties of Gases

    Gas properties can be modeled using math. Model that depends on

    V = volume of the gas (L)

    T = temperature (K)

    ALL temperatures MUST be in Kelvin!!! No Exceptions!

    n = amount (moles)

    P = pressure(atmospheres)

  • 6PressurePressure is defined

    as a force per unit area ( P = F/A )

    The SI unit of pressure is the pascal (Pa)

  • 7PressurePressure of air is measured

    with a BAROMETER(developed by Torricelli in 1643)

    Hg rises in tube until force of Hg (down) balances the force of atmosphere (pushing up). (Just like a straw in a soft drink)

    P of Hg pushing down related to

    Hg density

    column height

  • 8PressurePressure of liquid is

    measured with an assist of mass and volume or density of liquid

    m = mass

    V = volume

    d = density

    P = g x h x d (1.1)

  • 9Pressure

  • 10PressureColumn height measures

    Pressure of atmosphere

    1 standard atmosphere (atm) *

    = 760 mm Hg (or torr) *

    = 29.92 inches Hg *

    = 14.7 pounds/in2 (psi)*Termodynamics only

    = 101.3 kPa (SI unit is PASCAL)

    = about 34 feet of water!

    * Termodynamics only

    * Memorize these!

  • 11

    Pressure Conversions

    A. What is 475 mm Hg expressed in atm?

    1 atm

    760 mm Hg

    B. The pressure of a tire is measured as 29.4 psi.

    What is this pressure in mm Hg?

    760 mm Hg

    14.7 psi = 1.52 x 103 mm Hg

    = 0.625 atm475 mm Hg x

    29.4 psi x

  • 12

    Pressure Conversions

    A. What is 2 atm expressed in torr?

    B. The pressure of a tire is measured as 32.0 psi.

    What is this pressure in kPa?

  • 13

    Learning Check

    A gas has a volume of 675 mL at 35C and 0.850 atm pressure. What is the temperature in C when the gas has a volume of 0.315 L and a pressure of 802 mm Hg?

  • 14

    One More Practice Problem

    A balloon has a volume of 785 mL on a fall day when the temperature is 21C. In the winter, the gas cools to 0C. What is the new volume of the balloon?

  • 15And now, we pause for this

    commercial message from STP

    OK, so its really not THIS kind of STP

    STP in chemistry stands for

    Standard Temperature and


    Standard Pressure =

    1 bar (or an

    equivalent, 105 Pa)


    Temperature = 0 deg

    C (273 K)

    STP allows us to

    compare amounts of

    gases between different

    pressures and


  • 16

    Try This One

    A sample of neon gas used in a neon sign has a volume of 15 L at STP. What is the volume (L) of the neon gas at 2.0 atm and 25C?

  • 17


    Brings together gas properties.

    Can be derived from experiment and theory.


    P V = n R T

  • 18

    Using PV = nRTP = Pressure

    V = Volume

    T = Temperature

    N = number of moles

    R is a constant, called the Ideal Gas Constant

    Instead of learning a different value for R for all the possible unit combinations, we can just memorizeone value and convert the units to match R.

    R = 0.0821L atm

    Mol K

  • 19

    Using PV = nRTHow much N2 is required to fill a small room

    with a volume of 960 cubic feet (27,000 L) to 745 mm Hg at 25 oC?


    1. Get all data into proper units

    V = 27,000 L

    T = 25 oC + 273 = 298 K

    P = 745 mm Hg (1 atm/760 mm Hg) = 0.98 atm

    And we always know R, 0.0821 L atm / mol K

  • 20

    Using PV = nRTHow much N2 is reqd to fill a small room with a volume of 960

    cubic feet (27,000 L) to P = 745 mm Hg at 25 oC?


    2. Now plug in those values and solve for the unknown.

    PV = nRT

    n = (0.98 atm)(2.7 x 10 4 L)

    (0.0821 L atm/K mol)(298 K)

    n = 1.1 x 103 mol (or about 30 kg of gas)

    RT RT

  • 21

    Learning Check

    Dinitrogen monoxide (N2O), laughing gas, is used by dentists as an anesthetic. If 2.86 mol of gas occupies a 20.0 L tank at 23C, what is the pressure (mm Hg) in the tank in the dentist office?

  • 22

    Learning Check

    A 5.0 L cylinder contains oxygen gas at 20.0C and 735 mm Hg. How many grams of oxygen are in the cylinder?

  • 23Mixtures of Gases


    The % of gases in air Partial pressure (STP)

    78.08% N2 593.4 mm Hg

    20.95% O2 159.2 mm Hg

    0.94% Ar 7.1 mm Hg

    0.03% CO2 0.2 mm Hg

    PAIR = PN + PO + PAr + PCO = 760 mm Hg2 2 2

    Total Pressure 760 mm Hg

    Gases in the air

  • 24Daltons Law of Partial Pressures

  • 25Daltons Law of Partial Pressures

    2 H2O2 (l) ---> 2 H2O (g) + O2 (g)

    0.32 atm 0.16 atm

  • 26

    Daltons Law

    John Dalton


  • 27Health NoteWhen a scuba diver is several

    hundred feet under water, the

    high pressures cause N2 from

    the tank air to dissolve in the

    blood. If the diver rises too

    fast, the dissolved N2 will form

    bubbles in the blood, a

    dangerous and painful

    condition called "the bends".

    Helium, which is inert, less

    dense, and does not dissolve

    in the blood, is mixed with O2in scuba tanks used for deep


  • 28Soal-soal

    1. After the gases mix, what is the partial pressure of each gas, and what is the total pressure?

    2. What is the mole fraction of each gas in the mixture?

    3. Assume that unpolluted air has the composition shown

    in Table above

    (a) Calculate the number of molecules of N2, of O2, of Ar

    and of CO2 in 1.00 L of air at 21C and 1.00 atm.

    (b) Calculate the mole fractions of N2, of O2, of Ar and of

    CO2 in the air

  • 294. Let us represent gasoline as octane, C8H18. When hydrocarbon fuels burn in the presence of sufficient oxygen, CO2 is formed.

    Reaction A: 2C8H18 + 25O2 16CO2 + 18H2O

    But when the supply of oxygen is limited, the poisonous gas carbon monoxide, CO, is formed.

    Reaction B: 2C8H18 + 17O2 16CO + 18H2O

    Any automobile engine, no matter how well tuned, burns its

    fuel by some combination of these two reactions. Suppose an automobile engine is running at idle speed in a closed garage with air volume 97.5 m3. This engine burns 95.0% of its fuel by reaction A, and the remainder by reaction B.

    (a) How many liters of octane, density 0.702 g/mL, must be burned for the CO to reach a concentration of 2.00 g/m3?

    (b) If the engine running at idle speed burns fuel at the rate

    of 1.00 gal/h (0.0631 L/min), how long does it take to reach the CO concentration in (a)?

  • 30Collecting a gas over waterGases, since they mix with other gases readily, must be collected in an environment where mixing can not occur. The easiest way to do this is under water because water displaces the air. So when a gas is collected over water, that means the container is filled with water and the gas is bubbled through the water into the container. Thus, the pressure inside the container is from the gas AND the water vapor. Daltons Law of Partial Pressures becomes useful here.

  • 31

    Table of Vapor Pressures for Water

  • 32

    Solve This!

    A student collects some hydrogen gas over water at 20 degrees C and 768 torr. What is the pressure of the H2 gas?

    768 torr 17.5 torr = 750.5 torr

  • 33






    22.4 L of ANY gas

    AT STP = 1 mole

  • 34

    Gases and Stoichiometry

    2 H2O2 (l) ---> 2 H2O (g) + O2 (g)

    Decompose 1.1 g of H2O2 in a flask with a volume of 2.50 L. What is the volume of O2at STP?

    Bombardier beetle

    uses decomposition

    of hydrogen peroxide

    to defend itself.

  • 35

    Gases and Stoichiometry2 H2O2 (l) ---> 2 H2O (g) + O2 (g)

    Decompose 1.1 g of H2O2 in a flask with a volume of 2.50 L. What is the volume of O2 at STP?


    1.1 g H2O2 1 mol H2O2 1 mol O2 22.4 L O2

    34 g H2O2 2 mol H2O2 1 mol O2

    = 0.36 L O2 at STP

  • 36

    Gas Stoichiometry: Practice!

    A. What is the volume at STP of 4.00 g of CH4?

    B. How many grams of He are present in 8.0 L of gas at


  • 37

    What if its NOT at STP?

    1. Do the problem like it was at STP. (V1)

    2. Convert from STP (V1, P1, T1) to the stated conditions (P2, T2)

  • 38

    Try this one!

    How many L of O2 are needed to react 28.0 g NH3 at 24C and 0.950 atm?

    4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g)

  • 39

    CalculationP1 = 0.800 atm V1 = 180 mL T1 = 302 K

    P2 = 3.20 atm V2= 90 mL T2 = ??

    P1 V1 P2 V2= P1 V1 T2 = P2 V2 T1

    T1 T2

    T2 =

    T2 =

    T2 = 604 K - 273 = 331 C

    = 604 K

    P2 V2 T1

    3.20 atm x 90.0 mL x 302 K

    P1 V1

    0.800 atm x 180.0 mL

  • 40

    The KineticMolecular Theory

    1. Gases consist of discrete molecules. The individual molecules are very small and are very far apart relative to their own sizes.

    2. The gas molecules are in continuous, random, straight-line motion with varying velocities.

    3. The collisions between gas molecules and with the walls of the container are elastic; the total energy is conserved during a collision; that is, there is no net energy gain or loss.

    4. At any given instant, only a small fraction of the molecules are involved in collisions. Between collisions, the molecules exert no attractive or repulsive forces on one another; instead, each molecule travels in a straight line with a constant velocity.

  • 41Molecular SpeedTotal kinetic energy per mole of gas 3RT /2

    Root-mean-square speed

  • 42Distribution of Molecular Speed

  • 43