Page 21 - A Working Method Approach For Introductory Physical Chemistry Calculations

Introduction to Physical Chemistry 5
As the pressure applied by the piston is increased, the volume of the
gas decreases (i.e. the space it occupies), if the temperature is kept
constant. This is Boyle’s Law - the volume of a definite mass of gas at
constant temperature is inversely proportional to its pressure.

i.e. Boyle’s Law: Voc l/p
or V = k/p, where k is a constant of proportionality.

Example: A sample of gas G used in an air conditioner has a
volume of 350 dm3 and a pressure of 85 kPa at 25°C. Determine
the pressure of the gas at the same temperature when the volume is
500 dm3.

Solution:
The gas is at constant temperature, and therefore Boyle’s Law can be
applied:
Hence initially, V1 = k/pl or k = p1 V1
Hence k = (85 kPa) x (350 dm3)
= 29750 kPa dm3
However finally, p2 = k/V2
= 29750/500
= 59.5 kPa
Answer : Final Pressure = 59.5 kPa

Charles’s Law
In contrast, if the pressure is kept constant, the volume of a definite
mass of gas will increase, if the temperature is raised. This is Charles’s
Law: the volume of a definite mass of gas at constant pressure is
directly proportional to its temperature.
i.e. Charles’s Law: V oc T
or V = kT, where k is a different constant of
proportionality.

Example: A sample of gas G occupies 200 cm3 at 288 K and 0.87
bar. Determine the volume the gas will occupy at 303 K and at the
I same pressure.