WS8 - Chem 142 Name...

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Unformatted text preview: Chem 142 Name ________________________________________ Worksheet 8: Weak Acids and Bases and Salts Summer 2011 Section ________________ 1. What is the pH of each of the following weak acid/base solutions? What is the percent dissociation of the acid/base? Step 1: Write balanced dissociation reactions for each the acid/base and water Step 2: Rank the species in order of strength (using the Ka) Step 3: Determine whether the water will contribute to the H+ or OH- concentrations (base on Ka values) Step 4: For the strongest acid/base, write the equilibrium expression and calculate the equilibrium concentrations (5% rule?) Step 5: If needed, repeat step 5 for the other species Step 6: For an acid calculate pH from the [H+]; for a base calculate pOH or [H+] from [OH-], then find pH a. 0.500 M formic acid, HCOOH (Ka = 1.77 x 10-4) b. 0.200 M citric acid, C6H10O8 or HCA (Ka = 7.45 x 10-4) c. 0.350 M methylamine, CH3NH2 (Kb = 4.38 x 10-4) 2. What is the pH of each of the following weak acid mixtures, or polyprotic acids? What is the concentration of each anion (or conjugate base) resulting from the dissociation of each acid (or each step of a polyprotic acid dissociation)? What is the percent dissociation of each acid? Step 1: Identify all major species in solution (including water) Step 2: Write balanced dissociation reactions for each species Step 3: Rank the species in order of strength (using the Ka) Step 4: For the strongest acid, write the equilibrium expression and calculate the equilibrium concentrations (5% rule?) Step 5: Repeat step 5 for all remaining acids in order of strength, using the 5% rule where appropriate Step 6: Calculate pH from the [H+] a. 2.00 M formic acid, HCOOH (Ka = 1.77 x 10-4), and 1.50 M hypobromous acid, HOBr (Ka = 2.06 x 10-9) b. 1.40 M oxalic acid, H2C2O4 (Ka1=6.5 x 10-2, Ka2=6.1 x 10-5) Page 1 of 2 Chem 142 Summer 2011 3. Do you expect the following salts to be acidic, basic, or neutral? If acidic or basic, what is the pH of a 0.500 M solution of the salt? a. Na2C2O4 b. KI c. CH3NH3Cl d. NH4COOH (Kb (NH3) = 1.78 x 10-5) Page 2 of 2 ...
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This note was uploaded on 01/19/2012 for the course CHEM 142A taught by Professor Campbell during the Summer '11 term at University of Washington.

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