( 20 ) 20. A buffer with a pH = 10.5 is desired. Which of the following species

Question

(20) 20. A buffer with a pH = 10.5 is desired. Which of the following species would be the best choice for making this buffer, and which of the species would have to be in slight excess when the buffer is made? (For H3AsO4: Ka1 = 5.5 x 10-3; Ka2 = 1.7 x 10-7; Ka3 = 5.1 x 10-12)
H3AsO4
H3AsO4 in slight excess
H2AsO4-
H2AsO4- in slight excess
HAsO42
HAsO42 in slight excess
AsO43
AsO43 in slight excess (20) 20. A buffer with a pH = 10.5 is desired. Which of the following species would be the best choice for making this buffer, and which of the species would have to be in slight excess when the buffer is made? (For H3AsO4: Ka1 = 5.5 x 10-3; Ka2 = 1.7 x 10-7; Ka3 = 5.1 x 10-12)
H3AsO4
H3AsO4 in slight excess
H2AsO4-
H2AsO4- in slight excess
HAsO42
HAsO42 in slight excess
AsO43
AsO43 in slight excess

Explanation / Answer

pH = 10.5

if we form a buffer then

the equation

pH = pKa + log(conjugate/acid)

will help use

if pH = 10.5

we need a pKa near to that

pKA = -log(ka)

choose Ka3

pKa3 = -log(5.1*10^-12)= 11.292

then

pH = pKa + log(conjugate/acid)

10.5= 11.292+ log(conjugate/acid)

solve for ratio

(conjugate/acid) = 10^-(10.5-11.292)

(conjugate/acid) = 6.1944

we need much more conjugate than acid

H3AsO4 < H2AsO4-

therefore

answer is

H2AsO4- in slight excess

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