For each of the following solutions, calculate the initial pH and the final pH a
ID: 499365 • Letter: F
Question
For each of the following solutions, calculate the initial pH and the final pH after adding 0.0200 mol of NaOH. Part A For 220.0 mL of a buffer solution that is 0.235 M in HCHO 2 and 0.290 M in KCHO 2 , calculate the initial pH and the final pH after adding 0.0200 mol of NaOH( K a =1.8 10 4 ) . Part B For 220.0 mL of a buffer solution that is 0.2710 M in CH 3 CH 2 NH 2 and 0.2510 M in CH 3 CH 2 NH 3 Cl , calculate the initial pH and the final pH after adding 0.0200 mol of NaOH( K b =5.6 10 4 ) .
Explanation / Answer
Part A .
pKa = -log Ka = -log (1.8 x 10^-4)
= 3.74
pH = pKa + log [KCHO2 / HCHO2]
= 3.74 + log (0.290 / 0.235)
= 3.83
initial pH = 3.83
moles of KCHO2 = 0.290 x 220 / 1000 = 0.0638
moles of HCOOH = 0.0517
pH = pKa + log [salt + C / acid - C]
= 3.74 + log [0.0638 + 0.02 / 0.0517 - 0.02]
= 4.16
final pH = 4.16
Part B .
pKb = -log Kb = -log (=5.6 x 10^ 4)
= 3.25
pOH = pKb + log [salt / base]
= 3.25 + log [0.2510 / 0.2710]
= 3.22
initial pH = 10.78
moles of base = 0.05962
moles of salt = 0.05522
pOH = pKb + log [salt - C / base + C]
= 3.25 + log [0.05522 - 0.02 / 0.05962 + 0.02]
= 2.90
pH = 11.10
final pH = 11.10
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