37. Which salts of phosphoric acid are appropriate to use to make this buffer? (

Question

37. Which salts of phosphoric acid are appropriate to use to make this buffer? (circle the correct ones) What are you basing your selection on? H3PO4 NaH2PO4 Na2HPO4 Na3PO 38. You need to prepare a phosphoric acid buffer that has a buffer concentration of 0.1 M, a pH that is your target pH, and taking in to account activity. First, ignoring activity, calculate [A] and [HA] needed to prepare the buffer using the Henderson-Hasselbalch equation and the fact that [buffer] 0.1 M. (Hint: you have two equations and two unknowns). Show your work for you calculation below: 39. List all of the ions present in solution and their concentrations. Hint: There will be more ions present than in your monoprotic buffer prepared previously. 40. Calculate the ionic strength of the solution. Show your work.

Explanation / Answer

Buffer solution

37. The salt to prepare a buffer of pH 6.85 would be,

NaH2PO4

The pKa of this salt is close to the required pH of the solution

39. Ions present in buffer system,

Na+

H2PO4-

HPO4^2-

OH-

H+

This is a combination of NaH2PO4 and Na2HPO4 in the buffer solution. Some of the salt gets hydrolyzed.

40. Ionic strength of the buffer solution,

= 1/2(0.1 x 1^2 + 0.1 x 1^2 + 0.1 x 2 x 1^2 + 0.1 x 2^2)

= 0.4

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