Lecture 183/1/06

What is a buffer?

pH of a buffer system What is the pH of a buffer that is 0.12 M lactic acid HC 3 H 5 O 3 and 0.1 M sodium lactate? K a = 1.4 x 10 -4

pH using Henderson-Hasselbach Equation What is the pH of a buffer that is 0.12 M lactic acid HC 3 H 5 O 3 and 0.1 M sodium lactate? K a = 1.4 x 10 -4

Addition of strong acid/strong base Always a 2 step process:  Neutralization (100% completion)  Equilibrium

A buffer is prepared by mixing 0.15 mol of lactic acid and 0.2 mol of sodium lactate and raised to 1 L with water. K a = 1.4 x a) pH = ? b) pH after adding 0.05 mol HCl c) pH after adding 0.1 mol NaOH

So what determines pH of a buffer?

Buffer Capacity How much acid/base can be neutralized by a buffer with 0.25 M CH 3 COOH and 0.25 M of CH 3 COO - in 1 L.

Titration

Titration Curve Why do you use one? Equivalence point vs. Endpoint pH vs. acid/base added

Building a titration curve via calculations 4 Regions (to build one from calculations):  1) initial pH  2) before equivalence point  3) at equivalence point  4) after equivalence point

Building a titration curve via calculations Comparison of a titration of a strong acid vs. a weak acid Regionx-axis (mL of acid/base) 0.1 M NaOH y-axis (pH) 100 mL of 0.1 M HCl y-axis (pH) 100 mL of 0.1 M HF Initial pH Before the equivalence point (1/2 equiv. point) equivalence point After the equivalence point

Initial pH (NaOH titrating HCl) X-axis: Y-axis:

Building a titration curve via calculations Comparison of a titration of a strong acid vs. a weak acid Regionx-axis (mL of acid/base) 0.1 M NaOH y-axis (pH) 100 mL of 0.1 M HCl y-axis (pH) 100 mL of 0.1 M HF Initial pH 01 Before the equivalence point (1/2 equiv. point) equivalence point After the equivalence point

Equivalence point (NaOH titrating HCl) X-axis Y-axis

Building a titration curve via calculations Comparison of a titration of a strong acid vs. a weak acid Regionx-axis (mL of acid/base) 0.1 M NaOH y-axis (pH) 100 mL of 0.1 M HCl y-axis (pH) 100 mL of 0.1 M HF Initial pH 01 Before the equivalence point (1/2 equiv. point) equivalence point 1007 After the equivalence point

½ Equivalence point (NaOH titrating HCl) X-axis Y-axis

Building a titration curve via calculations Comparison of a titration of a strong acid vs. a weak acid Regionx-axis (mL of acid/base) 0.1 M NaOH y-axis (pH) 100 mL of 0.1 M HCl y-axis (pH) 100 mL of 0.1 M HF Initial pH 01 Before the equivalence point (1/2 equiv. point) equivalence point 1007 After the equivalence point

After Equivalence point (NaOH titrating HCl) X-axis Y-axis

Building a titration curve via calculations Comparison of a titration of a strong acid vs. a weak acid Regionx-axis (mL of acid/base) 0.1 M NaOH y-axis (pH) 100 mL of 0.1 M HCl y-axis (pH) 100 mL of 0.1 M HF Initial pH 01 Before the equivalence point (1/2 equiv. point) equivalence point 1007 After the equivalence point

Sulfanilic acid, which is used in making dyes is made by reacting aniline with sulfuric acid. The acid has a pK a value of The sodium salt of the acid is quite soluble in water. If you dissolve 1.25 g of the salt in 125 mL of solution, what is the pH of the solution?