GASES     Chapter 6

 

Gas

1. Least dense of the forms of matter
   Solid is more dense than liquid; liquid is more dense than gas

2. Properties
   A. Diffusion - move-a-bility of gas
             Gasoline or ether can be smelled rapidly.  Gas fumes move.
   B. Pressure - push-a-bility

The volume of a gas expands with heat and contracts with cold.     T8 V8      T9 V9 

If pressure is applied to a gas, the volume will decrease.      P8 V9            P9   T8

 

Gases

How many molecules in 1 mole of H2 gas?

How many atoms in 1 mole of H2 gas?

How many molecules in 1 mole of methane gas (CH4)?

How much does 1 mole of H2 gas weigh?

How much does 1 mole of CH4 gas weigh?

Review

A. 1 mole of every gas will contain Avagadro's number of molecules 6.02 x 1023.

B. The weight of 1 mole of gas will vary according to the molecular weight of the gas.

Volumes of Gases   

At standard temperature and pressure, 1 mole of gas will occupy 22.4 liters.  Standard temperature is 00 C and standard pressure is 1 atmosphere. 

Formula

    1 mole of gas @ STP           =      ___moles @ STP  
            22.4 L                                            L

 

What is the volume of 2 moles of O2 at STP?

What is the volume of 2 moles of CH4 at STP?

How much does 2 moles of O2 weigh?

How much does 2 moles of CH4 weigh?

ATMOSPHERIC PRESSURE     Page 168

 

Measure in mm Hg (mercury) 760 mm Hg at sea level.

  The air pushes on the Hg and makes it rise to the 760 mm mark.
  760 mm Hg is 1 atmosphere              
  29.9 inches Hg is 1 atmosphere
  760 Torr is 1 atmosphere   

  Denver has .8 atmospheric pressure. How many mm Hg would that   be equivalent to?

760 mm Hg = 760 Torr = 105 Pascals = 1 atm

GAS LAW FORMULAS

   P1V1    =    P2V2                                  PV  =  nRT
     T1              T2   

Effect of Pressure on Gas Volume       Pages 175 - 176     

The more pressure the smaller the volume - squeeze-a-bility of a gas

Boyle's Law     BP

At constant temperature the volume of a gas is inversely proportional to pressure.

 

 

Drill 

1. At 1 atm pressure the volume of a gas is 1 liter. What would be the volume at 2 atm pressure?    

Pressure inversely proportional to volume more pressure, less volume                 

            P9V\          P\ V9                  Visualize a balloon under pressure. It gets smaller.

 

P1 = 1 atm                P1V1        =   P2V2
P2  = 2 atm                   T1                 T2
V1 = 1 liter
V2 = x                                 

2. 1.5 liters of gas is under pressure of 2.5 atm. If the pressure is decreased to 0.75 atm, what is the new volume?

 

 

3. A container holds 500 ml of a gas at 25oC and 740 mm pressure. What will the volume of the gas be if the pressure is increased to 800 mm, temperature remaining constant?

 

4. A gas holds 2000 liters at 820 mm pressure. If the temperature remains constant, what will the volume of gas be at standard pressure?

  

Effect of Temperature on Gas Volume     Pages 177 - 180

 

Principle - the higher the temperature the more volume  needs.
Examples: tire and basketballs on cold days

Charles Law - at constant pressure, the volume of a given quanity of gas varies directly with absolute temperature.

How to do Problems

Think in terms of  K

Definition of Kelvin scale is the coldest it can get. There is no  -5K. All molecular activity stops at 0K.

1. 1.50 liters of gas at 25oC is heated to 350oC. The pressure remains the same. What is the new volume?

   a. The greater the heat the greater the volume.
   b. Think in K 

T1 = 25oC = 273 + 25 = 298K

T2 = 350oC = 273 + 350 = 623K
V1 = 1.51                                              
V2 = x                                                   

2. The volume of a gas at 210oC is 2.6 liters. What is the volume at -10oC if the pressure remains constant?

    T1 = 210oC = ______K         
    T2 = -10oC = _______K
    V1 = 2.6 L
    V2 =  x L

3. Ammonia gas at 50oC occupies 20 liters. What is its volume at 0oC pressure remaining constant?

 

4. What is the new volume when 1.4 liters of gas at 0.80 atm pressure and 20oC is changed to 1.5 atm pressure at 110oC?

  

Drill

1. The volume of a gas is 200 ml at 800 mm pressure. Calculate the volume of the same mass of gas at 765 mm.

 

2. A sample of gas occupies 1000 ml at 760 mm pressure. If the temperature remains constant, what would be the volume of the gas (a) at 100 mm pressure (b) at 500 mm pressure and (c) at 2.5 atm pressure?


 

3. The volume of a sample of oxygen is 5.0 liters at 722 mm and 20oC. What volume will the oxygen occupy at standard pressure and 20oC?

 

 

4. A mass of neon occupies 200 ml at 100oC. Find its volume at 0oC, the pressure remaining constant.

 

Gay-Lussac   Pages 162 - 163 

The pressure of a gas at a given volume is directly proportional to its temperature (K) with the volume remaining constant.

Have an airtight can - heat it.  What happens?

 

A can of air at 27E C and sea level is closed tightly.  It is heated to 327E C.  The volume is constant. What is the pressure on the inside of the can in Torr?

 

Recap

Boyle's Law  Volume is inversely proportional to the pressure
                                                  (temp constant)

Charles' Law  Volume is directly proportional to the temperature
                                                  (pressure constant)

Gay-Lussac's Law   Pressure is directly proportional to Temperature
                                                  (volume constant)

 

Combination of the three laws      

 

  P1V1   =          P2V2
                        -------                 -------
                                      T1                 T2

 A sample of O2 gas occupies a volume of 1.62 L at 755 mm Hg pressure and at a temperature of OE C.  What volume will this gas sample occupy at 725 mm Hg pressure and 50E C?

  

STP means standard temperature and pressure.
standard temp = 0oC (273K)
standard pressure = 1 atm (760 mm Hg)

 

Volume of a Mole of Gas

A mole of any gas at 0oC and 1 atm pressure (STP) occupies a volume of 22.4 liters. (1 mole = 6.02 x 1023 molecules)

1. What is the weight of 22.4 liters of O2 at STP?

 

2. What is the weight of 44.8 liters of O2 at STP?

Problem:
        How many moles of oxygen gas are present in 2 liters at 2 atm pressure and 27EC?  We could find out what the volume would be at STP ( 1Atm, 0EC) and then do a ratio.

 

P1  =  2  atm                                        P1V1     =    P2V2
P2  =   1 atm                                          T1               T2
V1  =  2 L
V2  =   x L                                            (2)(2)       =   (1) (x)
T1  =  27EC  + 273 =  300 K                 300                273        
T2  =  0E C  +  273 =  273 K     

                                                               300x  = 1092          x  = 3.64 L

                                      1 mole         =         x mole
                                     22.4 L                      3.64 L
                                         22.4 x  =  3.64              x  = 0.163 

If you were given the volume of a gas at a particular temperature and pressure, you could find the volume at STP.  Then you could find the number of moles of that gas using the 22.4 L = 1 mole.

There is another formula which is shorter.

PV  =   n R T         P = pressure in Atm   (if given in Torr change to Atm)
                                         V = volume in Liters
                                         n = number of moles
                                         R = constant  0.082 L atm/mol K
                                         T = temperature in EK

How many moles of oxygen gas are present in 2 liters at 2 atm pressure and 27EC?
(Would the answer be different for hydrogen gas?)

 

P  =  2 atm                                               PV   =   nRT
V  =  2 L                                               (2)(2)  =  n (0.082) (300)
T  =  27EC + 273 =  300 K                       4     =  24.6 x
n  =  x                                                      0.163 =  x
R  =  0.082 

How many grams of oxygen  are present in the example above?
If you had hydrogen gas instead of oxygen : 
a)  How many moles of hydrogen would you have?

 

b)  How many grams of hydrogen would you have?

 

How many grams of CO2 gas are there in a one liter container at 27E degrees C and 608 Torr?

 

A sample of C2H2 gas has a volume of 5.00 L at a pressure of 1.00 atm and a temperature of 100E C.  What will be the temperature of the gas, in degrees Celsius, if the volume is decreased to 1.00 L and the pressure is increased to 5.00 atm?

 

An adult human breathes in approximately 0.500 L of air at 36E C and 1.00 atm pressure with every breath.  What would this "breath volume" be at the same pressure if the temperature drops to 24E C?

 

 

DALTON'S LAW OF PARTIAL PRESSURES   Page 162

 

In a mixture of gases each gas acts independently from the other gases.  Daltons Law of partial pressures states that the total pressure of a mixture of gases is the sume of the individual pressures.

 

Pressure Total   =  Pressure of gas 1  + Pressure of gas 2   + Pressure of gas 3

 

To a tank already containing N2 at 2.0 atm pressure and O2 at 1.0 atm, we add an unknown quantity of CO2 until the pressure is 4.6 atm.  What is the partial pressure of the CO2?

Wednesday, March 21, 2007 01:28:03 PM