A. Consider the following equilibrium between ethanoic acid (K a = 1.8E-5 ) and
ID: 834199 • Letter: A
Question
A. Consider the following equilibrium between ethanoic acid (Ka = 1.8E-5) and its conjugate base:
(1) Will raising the pH shift the equilibrium to the right or to the left? to the right to the left
(2) Calculate the ratio of the concentration of acetate ion to the concentration of acetic acid at a pH of 7.23.
B. Consider the following equilibrium between propanoic acid (Ka = 1.3E-5) and its conjugate base:
(1) Will lowering the pH shift the equilibrium to the right or to the left? to the right to the left
(2) Calculate the ratio of the concentration of propanoate ion to the concentration of propanoic acid at a pH of 2.04.
C. Consider the following equilibrium between benzoic acid (Ka = 6.3E-5) and its conjugate base:
(1) Will lowering the pH shift the equilibrium to the right or to the left? to the right to the left
(2) Calculate the ratio of the concentration of benzoate ion to the concentration of benzoic acid at a pH of 1.81.
Explanation / Answer
A) raising the pH will shift the equilibrium to the right
pH = pKa + log [conjugate base] / [acid]
pKa = - log Ka = 4.745
=> 7.23 = 4.745 + log [conjugate base] / [acid]
=> [conjugate base] / [acid] = 305.68
B) Lowering the pH will shift the equilibrium to the left
pH = pKa + log [conjugate base] / [acid]
pKa = - log Ka = 4.886
=> 2.04 = 4.886 + log [conjugate base] / [acid]
=> [conjugate base] / [acid] = 1.425 x 10^-3
C) Lowering the pH will shift the equilibrium to the left
pH = pKa + log [conjugate base] / [acid]
pKa = - log Ka = 4.2
=> 1.81 = 4.2 + log [conjugate base] / [acid]
=> [conjugate base] / [acid] = 4.068 x 10^-3
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