9. Describe how to prepare a 1.00 L buffer solution from stock solutions of 2.00

Question

9. Describe how to prepare a 1.00 L buffer solution from stock solutions of 2.00 M NH3 and 1.50 M NH CI to have a pH of 9.50. Calculate the volumes, in mL, of NH, and NH CI that you would mix together to create 1 liter of buffer. The K, of NHs is 5.6 x 10 [Hint: One approach is to use the Henderson-Hasselbach equation to calculate the ratio of NHNH ion: another equation needed is the sum of the volumes of NHs and NH Cl is equal to 1.00 L Combining these two equations should lead to the answers.]

Explanation / Answer

pH of basic buffer =14 - (pkb+log(NH4Cl/NH3))

   pka= - logka

   = -log(5.6*10^-10)

         = 9.25

pkb of NH3 = 14-9.25 = 4.75

9.5 = 14 - (4.75+logx)

x = 0.562

NH4Cl/NH3 = 0.562

NH4Cl + NH3 = 1

0.562*NH3+NH3 = 1

NH3(1.562) = 1

volume of NH3 = 1/1.562 = 0.64 L

volume of NH4Cl must be taken = 1-0.64 = 0.36 L

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