Buffers and Titrations. Chapter 19. The Common Ion Effect & Buffer Solutions. Common ion effect - solutions in which the same ion is produced by two different compounds Buffer solutions - resist changes in pH when acids or bases are added to them due to common ion effect
Buffers and Titrations
For example ~ acetic acid CH3COOH and sodium acetate NaCH3COO
Ex. 1) Calculate the concentration of H+ and the pH of a solution that is 0.15 M in acetic acid and 0.15 M in sodium acetate. Ka = 1.8 x 10-5
Compare the acidity of a pure acetic acid solution and the buffer we just described.
Notice that [H+] is 89 times greater in pure acetic acid than in buffer solution.
Ex.2) Calculate the concentration of OH- and the pH of the solution that is 0.15 M in aqueous ammonia, NH3, and 0.30 M in ammonium nitrate, NH4NO3. Kb = 1.8 x 10-5
R NH3 + H2O NH4+ + OH-
I 0.15 0.30 0
C -x + x + x
E 0.15 –x 0.30 + x x
Let’s compare the aqueous ammonia concentration to that of the buffer described above.
Note, the [OH-] in aqueous ammonia is 180times greater than in the buffer.
Remember: pX = -log X
Ex. 3) If 0.020 mole of HCl is added to 1.00 liter of solution that is 0.100 M in aqueous ammonia and 0.200 M in ammonium chloride, how much does the pH change? Assume no volume change due to addition of the gaseous HCl.
1st ~ Calculate the pH of the original buffer solution
2nd ~ Calculate the concentration of all species after the addition of HCl.
3rd ~ Now that you have the concentrations of our salt and base, you can calculate the new pH.
4th ~ Calculate the change in pH.
Ex. 4) If 0.020 mole of NaOH is added to 1.00 liter of solution that is 0.100 M in aqueous ammonia and 0.200 M in ammonium chloride, how much does the pH change? Assume no volume change due to addition of the solid NaOH.
Ex. 5) Calculate the concentration of H+ and the pH of the solution prepared by mixing 200 mL of 0.150 M acetic acid and 100 mL of 0.100 M sodium hydroxide solutions.
Ex. 6) A) Find the number of moles of solid ammonium chloride, NH4Cl, that must be used to prepare 1.00 L of a buffer solution that is 0.10 M in aqueous ammonia, and that has a pH of 9.15
B) What mass is needed?
Ex. 7) Consider the titration of 100.0 mL of 0.100 Mperchloric acid with 0.100 M potassium hydroxide. Find the equivalence point of this rxn.
Salts of weak acids and strong bases hydrolyze to give basic solns so the soln is basic at the equivalence point and the soln is buffered before the equivalence point.