If you needed to make a buffer having a pH of 3.10, which of these mixtures of a

Question

If you needed to make a buffer having a pH of 3.10, which of these mixtures of an acid and a salt of its conjugate base would be the best choice?

Answer

Acid

Salt

Ka

A

HClO2(chlorous)

KClO2

1.2 x 10-2

B

HC2H2O2Cl(chloroacetic)

KC2H2O2Cl

1.3 x 10-3

C

HC3H5O2(propanoic)

KC3H5O2

1.3 x 10-5

D

HC2H3O2(acetic)

KC2H3O2

1.8 x 10-5

E

HOCl(hypochlorous)

KOCl

3.5 x 10-8

Answer

Acid

Salt

Ka

A

HClO2(chlorous)

KClO2

1.2 x 10-2

B

HC2H2O2Cl(chloroacetic)

KC2H2O2Cl

1.3 x 10-3

C

HC3H5O2(propanoic)

KC3H5O2

1.3 x 10-5

D

HC2H3O2(acetic)

KC2H3O2

1.8 x 10-5

E

HOCl(hypochlorous)

KOCl

3.5 x 10-8

Explanation / Answer

we should chose a component whose pKa is closer to required pH

A and B can both are closer
lets find their pKa

for A:
pKa = -log Ka
= -log (1.2*10^-2)
= 1.92

for B:
pKa = -log Ka
= -log (1.3*10^-3)
= 2.89

for C:
pKa = -log Ka
= -log (1.3*10^-5)
= 4.89

closest is B

Answer: B

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