What is the pKa for the acid HA if a solution of 0.65 M HA and 0.85 M NaA has a

Question

What is the pKa for the acid HA if a solution of 0.65 M HA and 0.85 M NaA has a pH of 4.75? 13. A) 5.50 14. A formic acid buffer containing 0.50 M HCOOH and 0.50 M HCOONa has a pH of 3.77. What will the pH be after 0.010 mol of NaOH has been added to 100.0 mL of the buffer? A) 3.67 B) 3.78 C) 3.81 D) 3.85 E) 3.95 15. Calculate [HyO'Toeal in a solution prepared by dissolving 0.17 mol acetic acid (CH3COOH) and 0.23 mol sodium acetate (CH:COONa) in enough water to produce 1.0 L of solution. K, for CHsCO0H is 1.8 x 10-. (a) [HOfeal 1.3x10- M (b) [H30tal 1.8 x 10s M (c) [H:0 Total 2.4 x 10-M d) [HsO Joeal 4.9 x 10-3M 16. Calculate the pH of a solution prepared by dissolving 0.20 mol of benzoic acid (abbreviated HOBz) and 0.15 mol of sodium benzoate (abbreviated NaOBz) in enough water to make 1.0 L of solution. The acid dissociation equilibrium constant for benzoie acid is Ks- 6.3 x 10-.

Explanation / Answer

13. As per Henderson-hasslebalch equation for buffers-

pH = pKa + log[conjugate base/acid]

Here,

pH = pKa + log[NaA/HA]

pKa = pH - log[NaA/HA]

pKa = 4.75 - log0.85/0.65

= 4.75 - 0.12

pKa = 4.63

Option B.

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