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					            Gas Pressure
• Gases exert pressure on any surface they
  come in contact with.

• Pressure is related to the number of
  collisions the gas molecules have with wall
  of a container per unit of area per unit of
  time.

• Pressure = Force / Area
                                             1
Elements that exist as gases at 250C and 1 atmosphere




                                                  2
                                                      5.1
           Physical Characteristics of Gases
•   Gases assume the volume and shape of their containers.
•   Gases are the most compressible state of matter.
•   Gases will mix evenly and completely when confined to
    the same container.
•   Gases have much lower densities than liquids and solids.




                                                            3
                                                                5.1
• The force of impact of a single
  collision is too small to be
  sensed. However, taken all
  together, this large number of
  impacts of gas molecules
  exerts a considerable force
  onto the surface of the
  enclosure: the gas pressure

• The larger the number of
  collisions per area of
  enclosure, the larger the
  pressure:
                                    4
           Force
Pressure =
           Area
 (force = mass x acceleration)




                                 Barometer   5
                                                 5.2
           Units for Pressure
• The SI-unit of pressure is Pascal [Pa]
Atmospheres (atm)
Millimeters of Mercury (mmHg)
Torr (torr)
Pressure per square inch (Psi) = lbs/in2

1atm = 760 mmHg = 760 torr
1 atm = 1.013 x105 Pa
1 atm = 14.69 psi

                                           6
Types of Pressure




                    7
              Boyles Law Demo
 lets assume that the balloon is tight, so that the amount or mass
    of air in it stays the same
• Density = mass/ volume,
• the gas density of the balloon thus varies only with its volume
    (when mass is held constant).
• If we squeeze the balloon, we compress the air and two things
    will happen:
     – the air pressure in the balloon will increase.
     – the density of the air in the balloon will increase.

• Since density is mass over volume, and the mass stays
  constant, the rise in density means that the volume of the
  balloon decreases: pressure goes up (↑); volume goes
  down (↓)

• Pressure and volume are inversely proportional                 8
                Boyle’s Law
 P↓
                                                    P↑

 V↑
                                                    V↓

At a constant temperature and a fixed quantity of gas
pressure and volume are inversely proportional.
           P1 V1 = P2 V2 ( 1 = initial 2 = final)        9
        Graphical Explanation




At a constant temperature and a fixed quantity of gas
pressure and volume are inversely proportional.
           P1 V1 = P2 V2 ( 1 = initial 2 = final)       10
  Boyle’s Example


Mini Lab




                    11
                    Chemistry in Action:
                 Scuba Diving and the Gas Laws
    Depth (ft)   Pressure
                  (atm)
        0           1

       33           2

       66           3




P                           V

                                                 12
                                                      5.6
                  Example
• A 3.0L bulb containing He at 145 mmHg is
  connected by a value to a 2.0 L bulb
  containing Ar at 355 mmHg. Calculate the
  partial pressure of each gas and the total
  pressure after the valve between the
  flasks is opened.



          3.0 L         2.0 L Ar
          He 145 mmHg   355 mmHg           13
                 Answer
First we need to find the total volume of the
  bulbs:
         Vtotal = Va +Vb = 3+2=5
Next we need to do Boyles’s law twice, once
  for each bulb to find P of each gas.
                 P1 V1 = P2 V2

He: 145(3) = P2 (5)   Ar = 355 (2) = P2 (5)
    P2 =87 mmHg           P2= 142 mmHg        14
               Answer Cont.
He: 145(3) = P2 (5)   Ar: 355 (2) = P2 (5)
    P2 =87 mmHg           P2= 142 mmHg

Now we need to find the after the valve between
 the two flasks is opened.

Ptotal= P2He +P2Ar
     = 87 + 142
      = 229 mmHg

                                                  15
               Bonus
Convert 229 mmHg to atm

229 mmHg x    1 atm       = .303 atm
             760 mmHg




                                       16
           Charles's Law Demo
• By warming the balloon up,
  we increase the speed of
  the moving gas molecules
  inside it.

• This increases the rate at
  which the gas molecules hit
  the wall of the balloon.

• Because the balloon’s skin
  is elastic, it expands upon
  this increased pushing from
  inside, and the volume
  taken up by the same mass
  of gas increases with         17
  temperature.
               Charles's Law
• At constant pressure the volume of gas is
  direct proportional to its temperature.
             V1 = V2           Note: Temp is ALWAYS in
                               Kelvin!!!!
             T1     T2


 T↓                                          T↑


 V↓                                          V↑      18
Charles's Law Mini Lab



     Mini Lab




                         19
Graphical Explanation




                        20
      Charles's Law Example
A balloon is filled to a volume of
7.00 x 102ml at a temperature of 20.◦C. The
  balloon is then cooled at a constant
  pressure to a temperature of 1.0x102K.
What is the final volume of the balloon?




                                              21
               Answer
20C + 273 = 293 K



7.00 x 102ml =    V2
293 K          1.0x102K.

V2= 238.9 ml

                           22
           Avogadro’s Law
• Equal volumes of gas at the same
  temperature and pressure contain equal
  numbers of moles.

• If temperature and Pressure are constant
  Volume of a gas is directly proportional to
  the number of moles.


                                                23
             Avogadro’s Law
V↓                                             V↑


                                               n↑
n↓
     At constant temperature and Pressure the
     Volume of a gas is directly proportional to
     the number of moles.
                       V1 = V2                      24
                       n1 n2
                    Avogadro’s Law
 V a number of moles (n)
                                 Constant temperature
 V = constant x n                Constant pressure

V1 / n1 = V2 / n2




                                                25
                                                     5.3
                Example
• How many liters of O2 gas are required to
  prepare 100 L of CO2 gas by the following
  reaction.

2 CO (g) + O2 (g)             2 CO2 (g)




                                          26
           Answer
           V1 = V2
           n1 n2


100 = V2
2 1
V2= 50L

                     27
                Homework
Chang: pg 210-211
#”S : 13,15, 18,19,21,23



BL
     3,17, 19, 20, 21


                           28
        Combined Gas Law
• This is used when nothing is constant in
  an experiment.

            P1V1    = P2V2
             T1        T2




                                             29
               Example
A gas is contained in a cylinder with a
  temperature of 281 K and a volume of 2.1
  ml at a pressure of 6.4 atm. The gas is
  heated to a new temperature of 298 K and
  the pressure decreases to 1 atm.
What is the new volume of the gas.



                                         30
                  Answer
         P1V1   = P2V2
          T1       T2

P1= 6.4atm
                     6.4 (2.1) = 1 (V2)
V1 = 2.1 ml
                         281      298
T1 = 281 k
P2= 1 atm
V2= ?                V2 = 14ml

T2 = 298 K
                                          31
           Ideal Gas Law 10.4
Ideal gas: a hypothetical gas whose pressure (atm),
  volume (L), and temperature (K) behave as predicted
  every time. (Perfect like each and everyone of you!)

One can visualize it as a collection of perfectly hard
  spheres which collide but which otherwise do not
  interact with each other.

Ideal Gas Law:     PV = nRT

gas constant:
R= 0.0821 L x Atm/mol x K

                                                         32
          Temperature and KE
• Temperature is a      %
  measure of the                       High
  Average kinetic       o              temp.
  energy of the          f
  molecules of a        M              Low
  substance.            ol             temp.
• Higher temperature     e
  faster molecules.      c
                        u
• At absolute zero (0   le
  K) all molecular       s
  motion would stop.
                             Kinetic
                                          34
                             Energy
    Kinetic Molecular Theory
• The Kinetic Molecular Theory explains
  the forces between molecules and the
  energy that they possess.

This theory has 3 basic assumptions.




                                          35
                 KMT
1.
Matter is composed of small particles
   (molecules). The measure of space that
   the molecules occupy (volume) is
   derived from the space in between the
   molecules and not the space the
   molecules contain themselves.


                                        36
• The molecules are in constant motion. This
  motion is different for the 3 states of matter.

Solid - Molecules are held close to each other by their
  attractions of charge. They will bend and/or vibrate,
  but will stay in close proximity.

Liquid - Molecules will flow or glide over one another,
  but stay toward the bottom of the container. Motion is
  a bit more random than that of a solid.

Gas - Molecules are in continual straightline motion. The
 kinetic energy of the molecule is greater than the
 attractive force between them, thus they are much
 farther apart and move freely of each other.
                                                      37
                 KMT 3
• When the molecules collide with each
  other, or with the walls of a container,
  there is no loss of energy.

• Random Fact:
The Average speed of an oxygen molecule
  is 1656 km/hr at 20ºC

                                             38
                Example
• A 50.0L cylinder of acetylene C2H2 has a
  pressure of 17.1 atm at 21C . What is the
  mass of acetylene in the cylinder.




                                              39
                   Answer
               PV = nRT

21 + 273 = 294 K
            17.1 (50.0) = n (0.0821) (294)
                     n = 35.4 mol
Need answer in grams

• 35.4 C2H2 x 26g C2H2 = 920 g C2H2
              1 mol C2H2

                                             40
      A short Way to do that
• mw = mRT = dRT
       PV     P

An unknown gas weighs 34g and occupies
 6.7L at a pressure of 2 atm at temperature
 of 245K.What is its average molecular
 weight.

                                          41
             Answer
• 51 g/mol




                      42
• Density = mass = P mw
            vol     RT




                          43
                  STP
• Standard Temperature and Pressure

• P = 1 atm = 760 torr
• T = 273 K
• Molar volume of ideal gas at STP = 22.42L




                                          44
      What is the volume (in liters) occupied by 49.8 g of HCl
      at STP?
                        T = 0 0C = 273.15 K
               Given:
                         P = 1 atm
PV = nRT
                                       1 mol HCl
     nRT                 n = 49.8 g x             = 1.37 mol
 V=                                   36.45 g HCl
      P
                            L•atm
      1.37 mol x 0.0821     mol•K
                                    x 273.15 K
 V=
                       1 atm

 V = 30.6 L

                                                          45
                                                               5.4
     Argon is an inert gas used in lightbulbs to retard the
     vaporization of the filament. A certain lightbulb
     containing argon at 1.20 atm and 18 0C is heated to
     85 0C at constant volume. What is the final pressure of
     argon in the lightbulb (in atm)?

PV = nRT       n, V and R are constant
nR   P
   =   = constant            P1 = 1.20 atm   P2 = ?
 V   T
                             T1 = 291 K      T2 = 358 K
P1   P2
   =
T1   T2
           T2
 P2 = P1 x    = 1.20 atm x 358 K = 1.48 atm
           T1              291 K
                                                          46
                                                               5.4
Dalton’s Law of Partial Pressure
• The sum of the partial pressures of the
  system is equal to the total pressure of the
  system.



• Ptotal = P1 + P2 + P3



                                             47
     Dalton’s Law of Partial Pressures


                           V and T
                             are
                           constant




P1                  P2            Ptotal = P1 + P2
                                             48
                                                  5.6
                   Homework
Chang pg 211
DAY 1
 #’s 27,29,31,33

Day 2 #/’s 38,41,49,51-53,62,66,71



Write out the combined gas law and ideal gas law formula
  5 times each and then do problems

23,27,28, 38

                                                           49

				
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