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Unformatted text preview: Chem 1622007 Hill & Petrucci Homework Chapter 15 (Note that my algebrasolving calculator requires my setting up equations involving ,X and lots of parentheses. Ignore these equations.) More AcidBase Equilibria 81. A solution is 0.405 M HCOOH (formic acid) and 0.326 M in the salt HCOONa (sodium formate). What is the pH of this buffer solution? HCOOH + H 2 O H 3 O + + HCOO HCOOH + H 2 O H 3 O + + HCOO Initial 0.405 M 0.326M Change Equilibrium HCOOH + H 2 O H 3 O + + HCOO Initial 0.405 M 0.326M ChangeX +X +X Equilibrium 0.405  X +X 0.326 + X K a = 1.8 x 104 Small K, so drop the Xs. HendersonHasselbalch: pH = pK a + log([A ]/[HA]) pH = log(1.8 x 104 ) + log([0.326]/[0.405]) = 3.65 83. What mass of (NH 4 ) 2 SO 4 (s) must be dissolved in 0.100 L of 0.350 M NH 3 (aq) to produce a buffer solution with pH = 10.05? NH 3 + H 2 O NH 4 + + OH NH 3 + H 2 O NH 4 + + OH Initial 0.350M Y Change Equilibrium pH = 10.05 HendersonHasselbalch: pH = pK a + log([A ]/[HA]) K b = K NH3 = 1.8 x 105 K ca = (1 x 1014 )/(1.8 x 105 ) = 5.56 x 1010 pH = log(5.56 x 1010 ) + log([0.350]/[Y]) 10.05 = log(5.56 x 1010 ) + log([0.350]/[Y]) (((10.05) = ((log(5.56 x 1010 )) + (log([0.350]/[Y])))),X) Y = 0.056M NH 4 + MW (NH 4 ) 2 SO 4 = 132.1gmol1 (NH 4 ) 2 SO 4 2NH 4 + + SO 4 2 0.056mol/L NH 4 + x (1 NH 4 ) 2 SO 4 /2 NH 4 + ) x 0.100L x 132.1 g/mol = 0.37g (NH 4 ) 2 SO 4 (s) 85. If 1.00 mL of 0.250 M HCl is added to 50.0 mL of the buffer solution described in Problem 81, what will be the pH of the final solution? From problem 81: pH = 3.65; therefore [H 3 O + ] = 2.24 x 104 HCOOH + H 2 O H 3 O + + HCOO Initial Change Equilibrium 0.405 2.24 x 104 0.326 0.250 mol/L x 0.001L = 0.000250 mol HCl added to 0.0500 L. Therefore 0.0050 mol HCl added to 1L. Therefore, new concentration of H 3 O + is 2.24 x 104 + 0.0050 = 5.224 x 103 M H 3 O + . Since more strong acid was added to the system at equilibrium, the reaction will shift to the left while going to completion. Reaction goes to completion because reactions of strong acids go to completion. HCOOH + H 2 O H 3 O + + HCOO Initial 0.405 (5.224 x 103 ) 0.326 Change +(5.224 x 103 )(5.224 x 103 )(5.224 x 103 ) Equilibrium 0.410 0.321 HendersonHasselbalch: pH = pK a + log([A ]/[HA]) pH = (log(1.8 x 104 )) + (log([0.321]/[0.410])) pH = 3.64 AcidBase Indicators 89. Why are so many more acidbase indicators suitable for the titration of a strong acid with a strong base than are suitable for the titration of a weak acid with a strong base? In comparing figures 15.15 and 15.16 one can see that the inflection part of the curve for the strong acid and strong base covers a pH range from approximately 3.5 to 11.5, whereas the inflection part of the curve for the weak acid and strong base covers a pH range of only approximately 7.0 to 11.0. There are more indicators available for 3.5 to 11.5 then for the limited 7.0 to 11.0....
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 Spring '08
 siegal

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