CHEMISTRY: THE MOLE-Gas laws

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(a)Gas laws

1. Matter is made up of small particle in accordance to Kinetic Theory of matter:

  Naturally, there are basically three states of matter: Solid, Liquid and gas:

  (i)A solid is made up of particles which are very closely packed with a definite/fixed shape and fixed/definite volume /occupies definite space. It has a very high density.

  (ii) A liquid is made up of particles which have some degree of freedom. It thus has no definite/fixed shape. It takes the shape of the container it is put. A liquid has fixed/definite volume/occupies definite space.

  (iii)A gas is made up of particles free from each other. It thus has no definite /fixed shape. It takes the shape of the container it is put. It has no fixed/definite volume/occupies every space in a container.

2.Gases  are affected by physical conditions. There are two physical conditions:

          (i)Temperature

          (ii)Pressure

3. The SI unit of temperature is Kelvin(K).

    Degrees Celsius/Centigrade(oC) are also used. 

 The two units can be interconverted from the relationship:

                             oC + 273= K

 K -273 = oC

Practice examples

1. Convert  the following into Kelvin.

(i) O oC

            oC  + 273  =  K substituting : O oC  +  273  =  273 K

 (ii) -273 oC

            oC  + 273  =  K substituting :  -273oC  +  273  = 0 K

(iii) 25 oC

            oC  + 273  =  K substituting : 25 oC  +  273  =  298 K

(iv) 100 oC

            oC  + 273  =  K substituting : 100 oC  +  273  =  373 K

2. Convert  the following into degrees Celsius/Centigrade(oC).

(i) 10 K

         K -273 = oC  substituting:     10   –   273   =  -263 oC

(ii) (i) 1 K

         K -273 = oC  substituting:     1   –   273   =  -272 oC

(iii) 110 K

         K -273 = oC  substituting:     110   –   273   =  -163 oC

(iv) -24 K

         K -273 = oC  substituting:     -24   –   273   =  -297 oC

The standard temperature is 273K = 0 oC.

The room temperature is assumed to be 298K  = 25oC

4. The SI unit of pressure is Pascal(Pa) / Newton per metre squared (Nm-2) .    Millimeters’ of mercury(mmHg) ,centimeters of mercury(cmHg) and atmospheres are also commonly used.

The units are not interconvertible but Pascals(Pa) are equal to Newton per metre squared(Nm-2).

The standard pressure is the atmospheric pressure.

Atmospheric pressure is equal to about:

          (i)101325 Pa

          (ii)101325 Nm-2

          (iii)760 mmHg

          (iv)76 cmHg

          (v)one atmosphere.

5. Molecules of gases are always in continuous random motion at high speed. This motion is affected by the physical conditions of temperature and pressure.

Physical conditions change the volume occupied by gases in a closed system.

The effect of physical conditions of temperature and pressure was investigated and expressed in both Boyles and Charles laws.

6. Boyles law states that

 “the volume of a fixed mass of a gas is inversely proportional to the pressure at constant/fixed temperature

Mathematically:

     Volume   α             1 (Fixed /constant Temperature)

                                Pressure

          V   α            1      (Fixed /constant T) ie PV = Constant(k)

                                P

From Boyles law , an increase in pressure of a gas cause a decrease in volume. i.e doubling the pressure cause the volume to be halved.

Graphically therefore a plot of volume(V) against pressure (P) produces a curve.

Graphically a plot of volume(V) against inverse/reciprocal of pressure (1/p) produces a straight line

For two gases then  P1 V1 =  P2 V2

P1  =   Pressure of gas 1

V1  =  Volume of gas 1

P2   =   Pressure of gas 2 V2   =   Volume of gas 2