Lesson 4

 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 T and n are controlled variables Charles’s Law p and n are controlled variables Pressure temperature law 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

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? G V 1 = 100 mL P 1 = 66.6 kPa V 2 = 250 mL S = 26.6 kPa RP 2 = ?P Therefore the pressure in the 250 mL container is 26.6 kPa. AP 1 V 1 = P 2 V 2

What if the temperature changes from 15° C to 20 °C ? G V 1 = 100 mL P 1 = 66.6 kPa T 1 = 20 °C = K V 2 = 250 mL T 2 = 15° C = S = 26.2 kPa RP 2 = ?P Therefore the pressure in the 250 mL container at 15° C is 26.2 kPa. A

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? G V 1 = 225 mL P 1 = 175 kPa T 1 = 75° C = T 2 = 20 °C = K P 2 =1.0 x 10 5 kPa S = mL RV 2 = ?P Therefore the new volume is mL. A

Questions  Page 438 # 26-34