Buffers and titrations
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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

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Buffers and titrations

Buffers and Titrations

Chapter 19


The common ion effect buffer solutions

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

  • Two common kinds of buffer solutions

  • solutions of a weak acid plus a soluble ionic salt of the weak acid

  • solutions of a weak base plus a soluble ionic salt of the weak base


Weak acids plus salts of weak acids

Weak Acids plus Salts of Weak Acids

For example ~ acetic acid CH3COOH and sodium acetate NaCH3COO


Buffers and titrations

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

  • (note: sodium acetate completely dissociates)

  • R CH3COOH + H2O  CH3COO- + H3O+

  • 0.15 0.150

  • -x +x+x

  • E. 0.15 – x0.15 + xx


Buffers and titrations

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.


Weak bases plus salts of weak bases

Weak Bases plus Salts of Weak Bases

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


Buffers and titrations

  • Substitute these values into the ionization expression for ammonia and solve algebraically.


Weak bases plus salts of weak bases1

Weak Bases plus Salts of Weak Bases

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.


Henderson hasselbach equation

Henderson-Hasselbach equation

For acids:

For bases:

Remember: pX = -log X


Buffering action

Buffering Action

  • Buffer 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.


Buffers and titrations

1st ~ Calculate the pH of the original buffer solution


Buffers and titrations

2nd ~ Calculate the concentration of all species after the addition of HCl.

  • HCl will react with some of the ammonia


Buffers and titrations

3rd ~ Now that you have the concentrations of our salt and base, you can calculate the new pH.


Buffers and titrations

4th ~ Calculate the change in pH.


Buffers and titrations

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.


Preparation of buffer solutions

Preparation of Buffer Solutions

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.

  • Determine the amounts of acetic acid and sodium hydroxide (before reaction)


Preparation of buffer solutions1

Preparation of Buffer Solutions

  • For 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?


Acid base indicators

Acid-Base Indicators

  • Equivalence point - point at which chemically equivalent amounts of acid and base have reacted

  • End point - point at which chemical indicator changes color


Common acid base indicators

Common Acid-Base Indicators


Strong acid strong base titration curves

Strong Acid/Strong Base Titration Curves

  • Titration 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.


Buffers and titrations

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.

  • Plot pH vs. mL of KOH added

  • 1:1 mole ratio


Strong acid strong base titration curves1

Strong Acid/Strong Base Titration Curves

  • Before titration starts the pH of the HClO4 solution is 1.00

    • Remember that perchloricacid is a strong acid


Buffers and titrations

  • After 20.0 mL of 0.100 M KOH has been added the new pH is 1.17.


Buffers and titrations

  • After 50.0 mL of 0.100 M KOH has been added the pH is 1.48.


Buffers and titrations

  • After 90.0 mL of 0.100 M KOH has been added the pH is 2.28.


Buffers and titrations

  • After 100.0 mL of 0.100 M KOH has been added the pH is 7.00.


Strong acid strong base titration curves2

Strong Acid/Strong Base Titration Curves

  • We’ve 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.


Weak acid strong base titration curves

Weak 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.


Strong acid weak base titration curves

Strong Acid/Weak BaseTitration Curves

  • Titration 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.


Weak acid weak base titration curves

Weak Acid/Weak BaseTitration Curves

  • Titration 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. 


Fun chem problem for you

Fun Chem problem for you

  • Blood 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?


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