The K_a of benzoic acid is 6.30 times 10^-5. The pH of a buffer prepared by comb

Question

The K_a of benzoic acid is 6.30 times 10^-5. The pH of a buffer prepared by combining 50.0 mL of 1.00 M potassium benzoate and 50.0 mL of 1.00 M benzoic acid is _____. A) 4.201 B) 0.851 C) 3.406 D) 1.705 E) 2.383 The pH of a solution prepared by mixing 50.0 mL of 0.125 M NaOH and 40.0 mL of 0.125 M HNO_3 is ____. A) 7.00 B) 13.29 C) 8.11 D) 11.00 E) none of the above The concentration of fluoride ions [F^-] in a saturated solution of barium fluoride BaF_2 is _____ M. The solubility product constant of BaF_2 is 1.7 times 10^-6. A) 1.5 times 10^-2 B) 1.4 times 10^-4 C) 3.8 times 10^-4 D) 3.0 times 10^-3 E) 7.5 times 10^-3 Which one of the following is always positive when a spontaneous process occurs? A) Delta H_surroundings B) Delta S_system C) Delta H_universe D) Delta S_surroundings E) Delta S_universe

Explanation / Answer

38)

Benzoic acid is a weak acid. General representation of its dissociates is

HA <-------> H+ + A-

Ka = [H+] [A-] / [HA]

Initially A- and HA are present. The change in the [H+] can be calculated using ICE table

[H+]

[A-]

[HA]

Initial

0

1 M

1 M

Change

+x

-x

-x

Equilibrium

x

1-x

1-x

Since x is very small quantity 1-x = 1

6.3 X 10-5 = x (1) / 1

x= 6.3 X 10-5

[H+]= 6.3 X 10-5

pH = -log 6.3 X 10-5 = 4.2006

Option A is the right answer

[H+]

[A-]

[HA]

Initial

0

1 M

1 M

Change

+x

-x

-x

Equilibrium

x

1-x

1-x

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