Question

A solution is made by mixing 36.0 mL of ethanol, C2H6O, and 64.0 mL of water. Assuming ideal behavior, what is the vapor pressure of the solution at 20

Answer 1

Vap pressure of ethanol at 20 degc is 59.3 mm Hg                                                                                
Vap pressure of water at 20 degc is 17.5 mm Hg                                                                          
                                                                                
Mol wt of ethanol is 46.07 g/mol                                                                                
Mol wt of water is 18                                                                           
                                                                                
Density of etanol is 0.789 gm / cc                                                                              
Densiity of water 1 gm / cc                                                                             
                                                                                
                                                                                
Given soln contains                                                                             
                                                                                
Moles of ethanol is 36*0.789/46.07 which is 0.62                                                                                
Moles of water is 64/18 which is 3.55 moles                                                                             
                                                                                
Total moles in given soln 4.17                                                                          
                                                                                
Mole fraction of ethanol = 0.62/4.17 which is 0.1486                                                                            
Mole fraction of water = 3.55/4.17 which is 0.8514                                                                              
                                                                                
By Raoult's law                                                                                 
                                                                                
Partial pressure of ethanol = 59.3*0.1486 which is 8.81 mm Hg                                                                           
Partial pressure of water = 17.5*0.8514 which is 14.9 mm Hg                                                                             
                                                                                
Therefore vap pressure of soln which is the sum of partial pressure of ethanol + and partial pressure of water                                                                          
stands at 8.81+14.9  = 23.71 mm Hg