Your supervisor has asked you to prepare 1.00 L of a buffer at pH 6.40 and said all necessary chemicals would be in the fume hood. When you get to the lab you find the following: 250 mL of 0.175 M acetic acid (Ka = 1.76 x 10-5) 350 mL of 0.215 M carbonic acid (Ka = 4.3 x 107) 2.5 M NaOH (stock bottle) 2.5 M HCl (stock bottle)

a) Which acid do you select? Explain.

b) Do you use NaOH or HCl to prepare the buffer? Explain.

c) How much NaOH or HCl do you need to prepare the buffer? Show all your work.

Answer:

Let us consider the acetic acid first and find out the moles of NaOH we will have in comparison to the moles of acetic acid

And now consider the carbonic acid and NaOH combination

Thus, it is clear that to use NaOH as the limiting reagent we have to use carbonic acid and prepare the buffer of desired pH.

(b) It is very clear that in order to make acidic buffer we should use strong base, so that strong base reacts with weak acid and form conjugate ion. Here, as both the available acids are weak, we should use a strong base in order to make the desired buffer.

(c) The volume of NaOH used will be 15.75 mL as shown in the calculation below.

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