Chapter 6: Unit 5. Combined Gas Law

Combined Gas Law

A: The combined gas law is an expression obtained mathematically combining Boyle’s and Charles’s laws. A change in pressure, temperature or volume that is brought about by changes in the other two variables can be calculated by using this law.

Mathematically we can write, when amount of gas (n) is constant,

Here is the graphical relationship between pressure, volume and temperature of a gaseous system.

Example:  A balloon contains 222 L of Helium at 25.⁰C and 760 mm of Hg. What is the volume of the balloon  when it reaches to an altitude where the temperature is -40⁰C and the pressure is 540 mm of Hg?

Ans: V2 is unknown.

222* 760/( 25+ 273)= 566.17= V2*540/(-40+ 273) , V2= 244.3 L or 244 L.

 Watch the following video:

Questions:

1. The pressure inside a 1.0 L balloon at 25⁰C was 750 mm of Hg. What is the pressure inside the balloon when it is cooled to -40⁰C and expands to 5.0 L volume?

Ans: 117 mm of Hg