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CEE 350 - Korshin - Winter 2012 - Practice Midterm(1)

# CEE 350 - Korshin - Winter 2012 - Practice Midterm(1) -...

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1 CLASS 350 PRACTICE MIDTERM EXAM Problem 1 The protonation constant of acetic acid CH 3 COOH is 4.8. Determine the contributions of the anionic and neutral forms of this compound at pH 3.8, 4.8. 5.8. Formulas to determine the speciation of a monoprotic acid are pK pH pH HA K H H ± ± ± ² ² 10 10 10 D pK pH pK A K H K ± ± ± ² ² 10 10 10 The contributions of the HAc and Ac - forms calculated using these formulas at the pH value given in the problem are shown in the table below pH Contribution of HAc Contribution of Ac - 3.8 90% 10% 4.8 50% 50% 5.8 10% 90% Problem 2 The solubility product K sp of a sparingly soluble solid barium sulfate BaSO 4 that consists of Ba 2+ and SO 4 2- ions is 10 -10 M 2 . Barium Ba 2+ is a toxic metal and its concentrations must not exceed 1 mg/L. What is the minimum concentration (in mg/L) of sulfate SO 4 2- that needs to be present in water to ensure that the concentration of barium does not exceed its toxicity limit? The atomic weights of barium, sulfur and oxygen are 138, 32 and 16 g/mol, respectively. The molar concentration of barium above which it must be removed is >@ ³´ M AW L mg C Ba Ba Ba 6 2 10 2 . 7 138000 1 / ± ² µ The molar concentration of sulfate that is necessary to suppress the concentration of barium to levels below this level is

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2 >@ ±² M Ba K SO sp 5 6 10 2 2 4 10 · 4 . 1 10 · 2 . 7 10 ³ ³ ³ ´ ³ The concentration of sulfate in mg/L units that is equivalent to this molar concentration is ±² ± ² ± ² L mg M mg M SO MW C SO SO / 3 . 1 / 000 , 96 · 10 · 4 . 1 10 · 2 . 7 10 · 5 6 10 2 4 4 4 ³ ³ ³ ³ Problem 3 The Henry law coefficient for ammonia gas is 58.8 M/(Atm·L). Determine the molar concentrations of hydrated ammonia [NH 3 ] when the partial pressure of ammonia gas is 10 -6 Atm. Also determine the total molar concentration of ammonia species (the sum of [NH 3 ]+[NH 4 + ]) in the solution with pH 8.2. The pK value of the deprotonation constant of [NH 4 + ] is 9.2. The equilibrium between ammonium ion and ammonia is described as shown below ´ ´ ´ l H NH NH 3 4 > @> @ ´ ´ ´ 4 3 4 NH H NH K NH The concentration of hydrate ammonia is defined by Henry law. >@ ±² ± ² M L M P K NH NH NH H 4 6 3 10 59 . 0 / 10 8 . 58 3 3 ³ ³ µ µ The molar concentration of NH 4 + can be determined based on the appropriate equilibrium > @ ´ ´ ´ 4 4 3 NH K NH NH H and > @> @ 4 2 . 9 5 2 . 8 3 4 10 9 . 5 10 10 9 . 5 10 4 ³ ³ ³ ³ ´ ´ µ µ µ ´ NH K NH H NH
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CEE 350 - Korshin - Winter 2012 - Practice Midterm(1) -...

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