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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

- 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

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

- (note: sodium acetate completely dissociates)
- R CH3COOH + H2O CH3COO- + H3O+
- 0.15 0.15 0
- -x +x +x
- E. 0.15 – x 0.15 + x x

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 buffer we just describedWeak 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

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

Weak Bases plus Salts of ammonia and solve algebraically.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- ammonia and solve algebraically.Hasselbach equation

For acids:

For bases:

Remember: pX = -log X

Buffering Action ammonia and solve algebraically.

- 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.

1 ammonia and solve algebraically.st ~ Calculate the pH of the original buffer solution

2 ammonia and solve algebraically.nd ~ Calculate the concentration of all species after the addition of HCl.

- HCl will react with some of the ammonia

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

4 ammonia and solve algebraically.th ~ Calculate the change in pH.

Ex. 4) ammonia and solve algebraically.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 ammonia and solve algebraically.

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 Solutions ammonia and solve algebraically.

- 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 ammonia and solve algebraically.

- 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 ammonia and solve algebraically.Indicators

Strong Acid/Strong Base ammonia and solve algebraically.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.

Ex. 7) Consider the titration of 100.0 ammonia and solve algebraically.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 ammonia and solve algebraically.Titration Curves

- Before titration starts the pH of the HClO4 solution is 1.00
- Remember that perchloricacid is a strong acid

- After 20.0 ammonia and solve algebraically.mL of 0.100 M KOH has been added the new pH is 1.17.

- After 50.0 ammonia and solve algebraically.mL of 0.100 M KOH has been added the pH is 1.48.

- After 90.0 ammonia and solve algebraically.mL of 0.100 M KOH has been added the pH is 2.28.

- After 100.0 ammonia and solve algebraically.mL of 0.100 M KOH has been added the pH is 7.00.

Strong Acid/Strong Base ammonia and solve algebraically.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 ammonia and solve algebraically.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 ammonia and solve algebraically.Titration 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 ammonia and solve algebraically.Titration 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 ammonia and solve algebraically.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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