Suppose you are performing an experiment that requires a constant pH of 9.00. 1)

Question

Suppose you are performing an experiment that requires a constant pH of 9.00.

1) Which buffer system would be most appropriate? HOBrNaOBr or HOClNaOCl? Ka for HOBr is 2.0×109, Ka for HOCl is 3.5×108.

For this question, the appropiate buffer is HOBR-NaOBr, though I'm not sure why.

2) Calculate the ratio of conjugate base to weak acid required to prepare a buffer with pH= 9.00.

I tried using the Henderson-Hasselbach equation for this but came up with the wrong answer according to my online homework. Any help would be appreciated!

Explanation / Answer

1) Buffer pka should be near pH then that buffer is suitable buffer

HOBr and NaOBr bufffer has HOBr Ka 2 x 10^-9 , pka = -log Ka = -log ( 2x10^-9) = 8.7

pka of second buffer HOCl is pka = -log ( 3.5x10^-8) = 7.456

hence 8.7 is nearer 9 hence HOBr/NaOBr is suitable buffer

2) pH = pka + log [NaOBr]/[HOBr]

9 = 8.77 + log [NaOBr]/[HOBr]

[NaOBr]/[HOBr] = 2

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