Chemistry 12

Chemical reactions also involve substances in the gas phase, therefore to determine the number of moles of a gaseous compound the ideal gas equation is used

 

                        PV = nRT                    where P → pressure in kPa

                                                                      V → volume in litres (L)

                                                                      n → number of moles

                                                                      T → temperature in Kelvin (K)

                                                                      R → universal gas constant; 8.314 kPa • L• mol^–1• K^–1

 

-the ideal gas law can be used to determine any one of the variables from the knowledge of the other three

 

Example:

A glass bulb with a volume of 225 mL contains 0.580 g of an unknown gaseous compound.
The pressure is measured as 145.60 kPa at a temperature of 25 ^oC. What is the molar mass of the compound?

 

Solution:

 

Convert 225 mL to L;  225 mL = 0.225 L

Convert 25 ^oC to Kelvin; 25 ^oC = 273 + 25= 298 K

Determine the number of moles of gas under the above conditions

 

n =PV = 145.0 kPa x 0.225 L

     RT     298 K x 8.314 kPa•L•mol^–1•K^–1

           = 1.32 x 10^–2mol

 

1.32 x 10^–2mol = 0.580 g

              ! mol  = x g

 

x g = 0.580 g x 1 mol  =  44.0 g/mol

        1.32 x10^–2mol

 

Activity - Solve the following problems:

 

1.      Calculate the volume of 36.0 g of steam at 115 ^oC and 110.0 kPa of pressure.

2.      If 28 g of N₂, nitrogen gas, occupy 22.4 L at STP calculate the volume of 7 g of N₂ at 0 ^oC and 202.6 kPa of pressure.

3.      If 6 g of a gas occupy 20 L at 40 ^oC and 303.9 kPa of pressure, find its molar mass.

4.      A tire with a volume of 3.60 L contains 0.367 moles of air at a pressure of 252 kPa. What is the temperature, in ^oC, of the air in the tire.