Use this information for the remaining problems. Phosphate buffer is common used

Question

Use this information for the remaining problems. Phosphate buffer is common used in biology or biochemistry experiments, but systems nvolving DNA often require high ithium or stabilize the many negative charges on the double Lithium and magnesium ons, however, cause phosphate to precipitate. See the Ko data below. To prevent precipitation, cacodylate buffer can be used in place of phosphate buffer for DNA experiments. Cacodylate, shown below, contains arsenic, but has a structure similar to phosphate and works very well as a phosphate replacement. The bonus is that you can keep a bottle of cacodylate buffer on the shelf for years and there will be no microbial OH As OH HO CH OH cacodylic acid phosphoric acid pKa 6.27 pKa 2.12 pKe3 12.32 CH phosphate 1.04 x 1024 262.8581 Mga (PO42 2.37 x 104 115.7951 Li3PO4

Explanation / Answer

14. Cacodylic acid

a. Volume of NaOH needed to reach equivalence point = 0.2 M x 50 ml/0.40 M

                                                                                        = 25 ml

b. Ka = 5.37 x 10^-7

cacodylic acid dissociates into cacodylate and H+

let x amount has disscoiated

then,

Ka = [cacodylate][H+]/[cacodylic acid]

5.37 x 10^-7 = x^2/0.200

x = [H+] = 3.28 x 10^-4 M

pH = -log[H+] = 3.48

c. pH at equivalence point

cacodylate + H2O <==> cacodylic acid + OH-

let x amount has hydrolyzed

Kb = Kw/Ka = [cacodylic acid][OH-]/[cacodylate]

[cacodylate] molarity = 0.2 M x 50 ml/75 ml = 0.13 M

1 x 10^-14/5.37 x 10^-7 = x^2/0.13

x = [OH-] = 4.98 x 10^-5 M

pOH = -log[OH-] = 4.30

pH = 14 - pOH = 9.70

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