probset4withanswers

probset4withanswers - Chem33-1, C-hour Problem Set 4...

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Chem33-1, C-hour Problem Set 4 Due Date: Oct. 26, 2007 Student name: ___________________ Student SISD #: __________________ 1. A buffer solution is prepared by dissolving 1 mole of formic acid (HCOOH, pKa = 3.75) and 0.5 mole of sodium formate (HCOONa) in enough water to make 1 L. a. Calculate the pH of this solution. b. 100 mL of a 1 M solution of HCl is added to the buffer solution. What is the pH? c. 100 mL of a 1 M solution of NaOH is added to the buffer solution. What is the pH? ANSWER: Recall that ] [ ] ][ [ 3 HCOOH O H HCOO Ka + = thus, ] [ ] [ ] [ 3 + = HCOO HCOOH Ka O H ] [ ] [ lg ] [ ] [ lg lg ] [ ] [ lg ] lg[ 3 + = = = = HCOO HCOOH pKa HCOO HCOOH Ka HCOO HCOOH Ka O H pH The pH is 45 . 3 5 . 0 1 lg 75 . 3 ] [ ] [ lg = = = HCOO HCOOH pKa pH b. After adding 100 ml of a 1M solution of HCl, the molar number of HCOOH in the buffer is mol ml L ml M mol 1 . 1 1000 1 100 1 1 = + , and the molar number of HCOO - is mol ml L ml M mol 4 . 0 1000 1 100 1 5 . 0 = , therefore 31 . 3 4 . 0 1 . 1 lg 75 . 3 ] [ ] [ log = = = HCOO HCOOH pKa pH c. After adding 100 ml of a 1M solution of NaOH, the molar number of HCOOH in the buffer is mol ml L ml M mol 9 . 0 1000 1 100 1 1 = , and the molar number of HCOO - is mol ml L ml M mol 6 . 0 1000 1 100 1 5 . 0 = + , therefore 57 . 3 6 . 0 9 . 0 lg 75 . 3 ] [ ] [ log = = = HCOO HCOOH pKa pH
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2. A sample of solid ammonium chloride was placed in an evacuated container and then heated so that it decomposed to ammonia gas and hydrogen chloride gas. After heating, the total pressure in the container was found to be 4.4 atm. Calculate Kp at this temperature for the decomposition reaction NH 4 Cl(s) NH 3 (g) + HCl(g) ANSWER: NH 4 Cl(s) NH 3 (g) + HCl(g) K p = P NH3 x P HCl For this system to reach equilibrium, some of the NH 4 Cl(s) decomposes to form equal moles of NH(g) and HCl(g) at equilibrium. Since mol HCl = mol NH 3 , then the partial pressures of each gas must be equal to each other. At equilibrium: P total = P NH3 + P HCl and P NH3 = P HCl . P total = 4.4 atm = 2 P NH3 , 2.2 atm = P NH3 = P HCl ; K p = (2.2 atm)(2.2 atm) = 4.8 atm 2 3. Changing the pressure in a rigid reaction vessel by changing the volume may shift the position of a gas-phase equilibrium, but changing the pressure by adding an inert gas
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probset4withanswers - Chem33-1, C-hour Problem Set 4...

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