Commercially 5 different b solutions of acetic acid (CH_3COOH) and sodium acetat

Question

Commercially 5 different b solutions of acetic acid (CH_3COOH) and sodium acetate (NaCH_3COO) are available. The pH of these can be described by the Henderson-Hasselbalch equation. For each of the buffer 0.20 mol of HCl is added to one liter of the solutions. For which of the following solutions will the pH change the most? 1.0 M CH_3COOH and 0.25 M NaCH_3COO 0.5 M CH_3COOH and 0.5 M NaCH_3COO 0.75 M CH_3COOH and 0.5 M NaCH_3COO 0.25 M CH_3COOH and 0.5 M NaCH_3COO 0.25 M CH_3COOH and 1.0 M NaCH_3COO

Explanation / Answer

pH will change the most for the LOWEST sum of total ions

so

a) 1+0.25 = 1.25

b) 0.5+ 0.5 = 1

c) 0.75+0.5 = 1.25

d) 0.25+0.5 = 0.75

e) 0.25+1.0 = 1.25

choose D:

d) 0.25+0.5 = 0.75

proof:

pH = pKa + log(A-/HA)

addition of 0.2 mol of H increases HA by 0.2 nd decreases 0.2

pH = 4.75 + log((0.5-0.2)/(0.25+0.2)) = 4.57

change

pH = 4.75-4.57 = 0.18

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