Buffers and Titrations

Chapter 19Buffers and TitrationsThe Common Ion Effect & Buffer Solutions2Common ion effect - solutions in which the same ion is produced by two different compoundsBuffer solutions - resist changes in pH when acids or bases are added to themdue to common ion effectTwo common kinds of buffer solutionssolutions of a weak acid plus a soluble ionic salt of the weak acidsolutions of a weak base plus a soluble ionic salt of the weak base2Weak Acids plus Salts of Weak AcidsFor example ~ acetic acid CH3COOH and sodium acetate NaCH3COO

33Ex. 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 10-54(note: sodium acetate completely dissociates)R CH3COOH + H2O CH3COO- + H3O+0.15 0.150-x +x+xE. 0.15 x0.15 + xx45

5Compare 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.

667Weak Bases plus Salts of Weak BasesEx.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 10-5R NH3 + H2O NH4+ + OH- I 0.15 0.30 0C -x + x + x E 0.15 x 0.30 + x x78Substitute these values into the ionization expression for ammonia and solve algebraically.

89Weak Bases plus Salts of Weak BasesLets compare the aqueous ammonia concentration to that of the buffer described above.Note, the [OH-] in aqueous ammonia is 180 times greater than in the buffer.

9Henderson-Hasselbach equation

For acids:For bases:Remember: pX = -log X 11Buffering ActionBuffer solutions resist changes in pH.

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.111st ~ Calculate the pH of the original buffer solution

132nd ~ Calculate the concentration of all species after the addition of HCl.HCl will react with some of the ammonia1314 3rd ~ Now that you have the concentrations of our salt and base, you can calculate the new pH.14154th ~ Calculate the change in pH.

1516Ex. 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.1617Preparation of Buffer SolutionsEx. 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.Determine the amounts of acetic acid and sodium hydroxide (before reaction)1718Preparation of Buffer SolutionsFor biochemical situations, it is sometimes important to prepare a buffer solution of a given pH.

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?1819Acid-Base IndicatorsEquivalence point - point at which chemically equivalent amounts of acid and base have reacted

End point - point at which chemical indicator changes color1920Common Acid-Base Indicators

2021Strong Acid/Strong Base Titration CurvesTitration curves are graphs that show the pH at various amounts of titrate added. Allows you to find the equivalence point.

For Titration curves, Plot pH vs. Volume of acid or base added in titration.

21Ex. 7) Consider the titration of 100.0 mL of 0.100 M perchloric acid with 0.100 M potassium hydroxide. Find the equivalence point of this rxn.Plot pH vs. mL of KOH added1:1 mole ratio

23Strong Acid/Strong Base Titration CurvesBefore titration starts the pH of the HClO4 solution is 1.00Remember that perchloric acid is a strong acid

2324After 20.0 mL of 0.100 M KOH has been added the new pH is 1.17.2425After 50.0 mL of 0.100 M KOH has been added the pH is 1.48.2526After 90.0 mL of 0.100 M KOH has been added the pH is 2.28.2627After 100.0 mL of 0.100 M KOH has been added the pH is 7.00.2728Strong Acid/Strong Base Titration CurvesWeve calculated only a few points on the titration curve. Similar calculations for the remainder of titration can show clearly the shape of the titration curve.

2829Weak Acid/Strong Base Titration Curves

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.2930Strong Acid/Weak BaseTitration CurvesTitration curves for Strong Acid/Weak Bases look similar to Strong Base/Weak Acid but they are inverted. The soln is buffered before the equivalence point and is acidic at the equivalence point. 3031Weak Acid/Weak BaseTitration CurvesTitration curves have very short vertical sections.

Solution is buffered both before and after the equivalence point.

Visual indicators cannot be used. Instead you can measure the conductivity in order to find the end point.

The math is complex, we will not worry about it in AP Chem. 3132Fun Chem problem for youBlood is slightly basic, having a pH of 7.35 to 7.45. What chemical species causes our blood to be basic? How does our body regulate the pH of blood?32Solution[H+]pH

0.15 M CH3COOH1.6 x 10-3 M2.80

0.15 M CH3COOH

&

0.15 M NaCH3COO1.8 x 10-5 M4.74

Solution[OH-]pH

0.15 M NH31.6 x 10-3 M11.20

0.15 M NH3&

0.15 M NH4NO39.0 x 10-6 M8.95

IndicatorColor in acidic rangepH rangeColor in

basic range

Methyl violetyellow0-2purple

Methyl orangepink3.1-4.4yellow

Litmusred4.7-8.2blue

Phenol-

phthaleincolorless8.3-10.0red