Question

The acid-dissociation constants of HC3H5O3 and CH3NH3+ are given in the table below. Which of the following mixtures is a buffer with a pH of approximately 3?
HC3H5O3

CH3NH3+

Ka

8.3 x 10-4

2.3 x 10-11



A mixture of 100. mL of 0.1 M NaC3H5O3 and 100. mL of NaOH
A mixture of 100. mL of 0.1 M CH3NH3Cl and 100. mL of CH3NH2
A mixture of 100. mL of 0.1 M HC3H5O3 and 50. mL of NaOH
A mixture of 100. mL of 0.1 M CH3NH3Cl and 50. mL of NaOH

Answer 1

Answer:

A mixture of 100. mL of 0.1 M HC3H5O3 and 50. mL of NaOH

Explanation:

The pH of a buffer solution is calculated using following relation

$pH=pKa+log(\frac{salt}{acid} )$

Thus the pH of buffer solution will be near to the pKa of the acid used in making the buffer solution.

The pKa value of HC₃H₅O₃ acid is more closer to required pH = 4 than CH₃NH₃⁺ acid.

pKa = -log [Ka]

For HC₃H₅O₃

pKa = 3.1

For CH₃NH₃⁺

pKa = 10.64

pKb = 14-10.64 = 3.36 [Thus the pKb of this acid is also near to required pH value)

A mixture of 100. mL of 0.1 M HC3H5O3 and 50. mL of NaOH

Half of the acid will get neutralized by the given base and thus will result in equal concentration of both the weak acid and the salt making the pH just equal to the pKa value.