all your acids and bases are belong to usacid-base

Buffers!I. First, the common ion effect A. This is the effect of adding an ion that is already in a reaction. 1. Example: In a solution of HF, you have this: a. HF(aq) ⇆ H+

(aq) + F-(aq)

2. If NaF were added these this would occur first:

a. NaF(aq) ⟶ Na+(aq) + F-

(aq)

b. Then, the F- would contribute to: HF(aq) ⇆ H+

(aq) + F-(aq)

B. In a mixture of two substances that would dissociate into a common ion, the effect is that the initial concentrations are changed. 1. The reaction will shift accordingly, reducing the dissociation of the acid/base 2. Be careful when writing your equations for these problems! C. Example: 1. What is the pH of a 0.10M soln. of acetic acid?

2. What is the pH of a mixture of 0.10M acetic acid and 0.10M of sodium acetate?

II. What is a buffer? A. A buffer solution resists change in pH (blood) B. How to make a buffer: 1. You must have significant quantities of: a. an acid and its conjugate base or b. a base and its conjugate acid C. The concentration of the acid/base and its conjugate pair must be within a factor of 10 of each other.III. Calculating the pH of buffers A. Every buffer problem involves some stoich to find the initial conc. B. Afterwards, the calculation is the same as usual. C. Example: What is the pH of a solution where 50. mL of 0.50M NaC2H3O2 is mixed with 25mL of 0.25M HC2H3O2?

All Your Acids and Bases are Belong to us...Acid-Base Equilibria Ch. 13-14

IV. A shortcut! A. The Henderson-Hasselbach (HHB) Equation: 1.

2. It only works for buffer solutions! 3. Let’s repeat our example and see what we get...

B. Example: Calculate the pH of the following solution: 25 mL of a 0.150M solution of hypochlorous acid (HOCl) and 32mL of a 0.45M potassium hypochlorite (KOCl).

C. Example: Calculate the pH when 25.0mL of 0.50M methylammonium nitrate is mixed with 75.0mL of 0.30M methylamine

D. When adding other items to a system: 1. Do the stoich to account for what happens once that item is added. 2. Determine whether it is a buffer or not a. if so, use HHB Equation. b. if not, use ICE chart. E. The pOH version:

TitrationsI. Reacting an acid/base with the opposite to find an unknown concentration. A. To do so, you must titrate to the equivalence point. B. This is the point where the moles of acid=moles of base. C. The pH of the equivalence point is not always 7! II. Two ways to reach the equivalence point: A. Colorimetrically: using indicators that change at the pH at the equivalence point (phenolphthalein) B. Titration curve: Using a pH meter.III. Titration curves A. Refer to titration handout. B. Know how to sketch a curve and qualitatively determine the pH based on what is reacting. C. The stronger the acid, the steeper the inflection.


D. A special point: when pH=pKa (very useful!!!) 1. This happens when (not the same as the equivalence point!): a. [acid]=[conjugate base] b. [base]=[conjugate acid] 2. When this is done, you can find the Ka using HHB!

E. Polyprotic acid curves (acids with more than one H+) 1. They have more than one inflection point therefore you have two equivalence points! 2. There are several points that you could use to find the the Ka.


IndicatorsI. Indicators are weak acids that have a conjugate base that produces a different color. A. HIn ⇌ H+ + In-

blue green B. Picking an appropriate indicator: 1. Pick one where the color change is close to the pH of the equivalence point. 2. Make sure the color change is noticeable at the equivalence point. C. The color change occurs at approximately the - log of the indicator’s Ka. (a.k.a. pKa) D. Therefore, pick an indicator where the pKa of the indicator=the pH of the equivalence point. E. The equivalence point of a titration is 4.3: Which of the following indicators would be a good choice? Explain your choice. Indicator Ka

Thymol Blue 2.3 x 10-2

Eriochrome Black T 5.4 x 10-5

Alizarin 6.6 x 10-6

m-Nitrophenol 8.3 x 10-8

Thymolphthalien 2.5 x 10-9

Alizarin Yellow R 4.3 x 10-11

Review...I. Toolbox: A. Switch b/w Ka to Kb, pH/pOH, [H+]/[OH-] B. ICE chart and Solver C. -log of quantities (pH, pOH, pKa, pKb) D. HHB equation (only for buffers!)II. Types of problems: A. pH of an acid/base B. pH of a salt C. pH of a buffer (HHB) D. pH AFTER you add something to a system (a.k.a. titration)III. Know... A. Acid/base, buffer, conjugate definitions B. Indicators C. Titration curves


all your acids and bases are belong to usacid-base

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