Lesson 4

COMBINED GAS LAW

 The combined gas law is not a new law but a combination of Boyle's and Charles' laws, hence the name the combined gas law.

 In short, this combined gas law is used when it is difficult to keep either the temperature or pressure constant.

  (do not copy) In many experiments with gases, keeping either the pressure or temperature constant is not even attempted.

Boyle’s Law Charles’s Law Pressure temperature law

T

and

n

are controlled variables

p

and

n

are controlled variables

v

and

n

are controlled variables

 If these formulas are combined, we find that the product of the pressure and volume of a gas sample is proportional to its absolute temperature in Kelvin;  k is a constant and so the formula can also be written

G 

Example 1

Helium in a 100 mL container at a pressure of 66.6 kPa is transferred to a container with a volume of 250 mL. What is the new pressure if no change in temperature occurs?

V 1 = 100 mL P 1 = 66.6 kPa V 2 = 250 mL S = 26.6 kPa R A P 2 = ?

P 1 V 1 = P 2 V 2 P Therefore the pressure in the 250 mL container is 26.6 kPa.

A R G

What if the temperature changes from 15° C to 20 °C ?

V 1 = 100 mL P 1 T 1 = 66.6 kPa = 20 ° C = 293.15 K S = 26.2 kPa V 2 T 2 = 250 mL = 15 ° C = 288.15 P 2 = ?

P Therefore the pressure in the 250 mL container at 15 ° C is 26.2 kPa.

 G

Example 2

A gas has a volume of 225 mL at 75 o C and 175 kPa. What will be its volume at a temperature of 20 o C and a pressure of 1.0 X 10 5 kPa?

S V 1 = 225 mL P 1 T 1 T 2 = 175 kPa = 75 ° C = 348.15

= 20 ° C = 293.15 K P 2 =1.0 x 10 5 kPa = 0.332 mL R V 2 = ?

P Therefore the new volume is 0.332 mL . A

Questions

 Page 438 # 26-34