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PRACTICE QUIZ KEY AQUEOUS EQUILIBRIA (20 points)

PRACTICE QUIZ KEY AQUEOUS EQUILIBRIA (20 points). 1. (2 pts) Which makes a pH 7 buffer?. a. 1 M NaOH and 1 M NaCl b. 1 M HC 2 H 3 O 2 and 1 MNaC 2 H 3 O 2 c. 1 M HCl and 1 M NaCl d. 1 M NH 4 Cl and 1 M NH 3. 2. (10 pts) Calculate the pH. 0.200 M x 0.0750 L = 0.0150 mol

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PRACTICE QUIZ KEY AQUEOUS EQUILIBRIA (20 points)

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  1. PRACTICE QUIZ KEY AQUEOUS EQUILIBRIA (20 points)

  2. 1. (2 pts) Which makes a pH 7 buffer? • a. 1 M NaOH and 1 M NaCl • b. 1 M HC2H3O2 and 1 MNaC2H3O2 • c. 1 M HCl and 1 M NaCl • d. 1 M NH4Cl and 1 M NH3

  3. 2. (10 pts) Calculate the pH. • 0.200 M x 0.0750 L = 0.0150 mol • Step 1: OH- + HCN W H2O + CN- • Init. 0.0150 0.0150 0 • RXN -0.0150 -0.0150 +0.0150 • Final 0 0 0.0150 mol • (Must be equal at final) • Change mol to M; 0.0150 mol / total volume • V of OH-: V = mol / M = 0.0150 mol / 0.300 M = 0.0500 L • Total volume = 50.0 mL OH- + 75.0 mL HCN = 125.0 mL = 0.125 L • 0.0150 mol CN- / 0.125 L = 0.120 M CN- • Step 2: CN- + H2O W HCN + OH- • 0.120 – x x x • Kb = Kw/Ka = 1.0 x 10-14/4.0 x 10-10 = 2.5 X 10-5 • Kb = [HCN][OH-]/[CN-] = x2 / 0.120 • x = 1.7 x 10-3 = [OH-]; pOH = 2.78; pH = 11.22

  4. 3. (8 pts) What is the pH? • HF W H+ + F- • At equil.: 0.800-x x 0.500+x • Assume “x” is small enough to ignore… • Ka = [H+][F-]/[HF] = 4.0 x 10-4 • 4.0 x 10-4 = x(0.500)/(0.800) • x = 6.4 x 10-4 = [H+] • pH = 3.19

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