Chemistry 2000, Fall 96, Assignment 3 Solutions

1. We first need to know how many moles of the acid and base we have:

   

   There is more than enough sodium hydroxide to neutralize the acid. The excess hydroxide is found by difference:

   

   (Note that the total volume after mixing the two solutions is 150mL.) To find the pH, we first calculate :

   

2. The of barium carbonate is . The solubility equilibrium is

   

   One barium ion is generated for each carbonate ion so

   

   The solubility is therefore . However we want our answer in g/L. The conversion is made with the molar mass of barium carbonate:

   

3. The temperature must increase from -20 to so we have

   

4. The water must first warm from 20 to :

   

   If 335kJ of heat is used to bring the water to its boiling point, the remaining heat (115kJ) goes into evaporating water. The latent heat of vaporization of water is 40.79kJ/mol. The number of moles of water vaporized is therefore

   

5. The 's of the two protons of are 7.0 and 19. It follows that in a pH 12 buffer, just about all of the sulfide will be in the form of . The solubility equilibrium is

   

   with an equilibrium constant

   

   The sulfide is converted to as it is formed by the equilibrium

   

   for which

   

   The overall reaction is therefore

   

   The equilibrium constant for this reaction is

   

   Since the pH of the buffer is 12, the activity of hydrogen ions is . Furthermore, the stoichiometry of the reaction results in one equivalent of being formed for each equivalent of :

   

   Thus the solubility of lead sulfide in a pH 12 buffer is .