CHARLES LAW:According to this law, the volume of a sample of a gas is directly proportional to its absolute Temperature (in K) at constant pressure. On increasing the Temperature the kinetic energy of the molecules increases, so they collide with the walls with greater force and because we are keeping the pressure constant this greater force results in increase of volume of the sample.

V ά T (K)

V/T = constant V = kT

V1/T1 = V2/T2 = V3/T3 = on

The variation of the volume of a fixed amount of gas with the temperature at constant pressure. Note that in each case the isobars extrapolate to zero volume at T = 0, or q = –273°C.
The V-T graph obtained is a straight line passing through origin as it represents the equation y = mx where m is the slope of the line. At 0 K the volume also becomes almost zero. The graph is extrapolated/dotted because this condition is unattainable. However, when the same graph is drawn with Temperature in 0C then the curve shifts and it intersects the x-axis at -273.150C as shown. These graphs are called “isobars”(constant pressure curves).

Q. A sample of gas at 1.20 atm and 27°C is heated at constant pressure to 57°C. Its final volume is 4.75 L. What was its original volume?

P1 = P2 = 1.2 atm
V1 =?                  V2 = 4.75L
T1= 300K            T2 = 330 K
From Charles’ law:
V1      =       V2
T1                    T2
 V1 = 4.32L

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