HMWK 5 FA 2015

# HMWK 5 FA 2015 - C211 Homework 5 Fall 2015 Name 13 points...

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C211 Homework 5 Fall 2015 Name _________________________ 13 points. Due Wednesday October 14 by 5 pm. Answer key will be posted to BB. No late papers will be accepted. 1. A solution is 0.00425 M in a diprotic organic acid H 2 A, at 25 o C. (Neglect ionic strength effects.) The first acidic H+ is strong (~100% dissociated), but the second is weak, with K a = 8.9 x 10 3 . (a) (0.1 pt) What is [H+] that results from the loss of the first proton, only? (Hint: no calculation necessary. Do this by inspection!) (b) (1 pt) Calculate the [H+] total of the solution at equilibrium. (Hint: make ICE table, solve quadratic) [H+] total = (c) (0.2 pts) Calculate the percent to which the second proton on the acid is dissociated. % dissociation = (d) (0.2 pt) Calculate pH of the solution. pH =

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2. (0.5 pts) (a) A buffer is prepared from ammonia, NH 3 , and ammonium chloride, NH 4 Cl, so that pH = 9.500. What ratio of conjugate base to conjugate acid must be used? K b for NH 3 = 1.75 x 10 5 Ratio = (b) (0.5 pts) Total analytical concentration C NH3 = 0.150 M, Calculate the actual concentrations of each partner in the buffer. [NH 4 + ] = [NH 3 ] = 3. (a) (1 pts) A pH 3.30 buffer is prepared from iodoacetic acid ICH 2 COOH (pK a = 3.175), and sodium iodoacetate, ICH 2 COO Na + . If ionic strength of the buffer solution must not exceed 0.200 M, what are the maximum concentrations of the two components that could be used? [ICH 2 COOH] = [ICH 2 COO Na + ] = (b) (1 pts) What volume of KOH (conc = 1.850 M), could be added to 50.00 mL of the buffer solution of part (a), before the pH would increase to 4.000? Attach extra paper as needed.
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