Chapter 19 Buffers and Titrations

The Common Ion Effect & Buffer Solutions 2 ______________- solutions in which the same ion is produced by two different compounds ______________- resist changes in pH when acids or bases are added to them –due to common ion effect Two common kinds of buffer solutions 1solutions of a ______________plus a soluble ____________________________ 2solutions of a ______________plus a soluble ____________________________

Weak Acids plus Salts of Weak Acids For example ~ acetic acid CH 3 COOH and sodium acetate NaCH 3 COO 3

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. K a = 1.8 x (note: sodium acetate completely dissociates) R CH 3 COOH + H 2 O  CH 3 COO- + H 3 O + I C.-x +x+x E – x xx

5

Compare the acidity of a pure acetic acid solution and the buffer we just described. Notice that [H + ] is ___ times greater in pure acetic acid than in buffer solution. 6

7 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, NH 3, and 0.30 M in ammonium nitrate, NH 4 NO 3. K b = 1.8 x R NH 3 + H 2 O  NH OH - I C -x + x + x E 0.15 –x x x

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

9 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 ____times greater than in the buffer.

Henderson-Hasselbach equation For acids: For bases: Remember:

11 Buffering Action __________________________________________. Ex. 3) If mole of HCl is added to 1.00 liter of solution that is M in aqueous ammonia and M in ammonium chloride, how much does the pH change? Assume no volume change due to addition of the gaseous HCl.

1 st ~ Calculate the pH of the original buffer solution

13 2 nd ~ Calculate the concentration of all species after the addition of HCl. HCl will react with some of the ammonia

14 3 rd ~ Now that you have the concentrations of our salt and base, you can calculate the new pH.

15 4 th ~ Calculate the change in pH.

16 Ex. 4) If mole of NaOH is added to 1.00 liter of solution that is M in aqueous ammonia and M in ammonium chloride, how much does the pH change? Assume no volume change due to addition of the solid NaOH.

17 Preparation of Buffer Solutions Ex. 5) Calculate the concentration of H + and the pH of the solution prepared by mixing 200 mL of M acetic acid and 100 mL of M sodium hydroxide solutions. Determine the amounts of acetic acid and sodium hydroxide (before reaction)

18 Preparation of Buffer Solutions For ______________ 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, NH 4 Cl, 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?

19 Acid-Base Indicators ____________________________- point at which chemically equivalent amounts of acid and base have reacted ______________- point at which chemical indicator changes color

20 Common Acid-Base Indicators

21 Strong Acid/Strong Base Titration Curves ______________are graphs that show the pH at various amounts of titrate added. Allows you to find the ______________. For Titration curves, Plot ______________of acid or base added in titration.

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

23 Strong Acid/Strong Base Titration Curves Before titration starts the pH of the HClO 4 solution is _____ Remember that perchloric acid is a strong acid

24 After 20.0 mL of M KOH has been added the new pH is _____.

25 After 50.0 mL of M KOH has been added the pH is _____.

26 After 90.0 mL of M KOH has been added the pH is ____.

27 After mL of M KOH has been added the pH is ____.

28 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 __________of the titration curve.

29 Weak Acid/Strong Base Titration Curves Salts of weak acids and strong bases hydrolyze to give basic solns so the soln is _______at the equivalence point and the soln is _____________before the ______________point.

30 Strong Acid/Weak Base Titration Curves Titration curves for Strong Acid/Weak Bases look similar to Strong Base/Weak Acid but they are inverted. The soln is _______before the equivalence point and is __________at the equivalence point.

31 Weak Acid/Weak Base Titration Curves Titration curves have ______________vertical sections. Solution is buffered both ______________and ______________the equivalence point. _______________________cannot be used. Instead you can measure the ______________ in order to find the end point. The math is complex, we will not worry about it in AP Chem.

32 Fun Chemistry problem for you Blood is slightly basic, having a pH of 7.35 to What chemical species causes our blood to be basic? How does our body regulate the pH of blood?