1. Assume the range of concentrations for the unknown acids is between 0.30 M an
ID: 529739 • Letter: 1
Question
1. Assume the range of concentrations for the unknown acids is between 0.30 M and 0.80 M and that you will be titrating a 5-mL aliquot of an unknown acid with 0.250 M NaOH.
a. What equivalence point volume (in mL) of NaOH would you expect if you were assigned the most dilute unknown acid? b. What equivalence point volume (in mL) of NaOH would you expect if you were assigned the most concentrated unknown acid?
2. Calculate the pH of a 25.00 mL sample of 0.500 M cinnamic acid solution. The pKa value for cinnamic acid is 4.45.
Explanation / Answer
Volume of unknown acid sample = 5 mL
a) most dilute solution has concentration = 0.30M
Thus at equivalence
mmoles of acid = mmoles of base
0.30M x 5 mL = 0.250M x V mL
thus V = 6 mL
At equivalence volume of NaOh = 6mL
b) similarly when [acid] = 0.80M
0.80M x 5 mL = 0.25M x v
And V = 16 mL
At equivalence volum of NaOH = 16 mL
2) [acid] = 0.5 0M
pKa of acid = 4.45
pH of acid = 1/2 [pKa -log C]
= 1/2 [4.45 -log 0.5]
= 2.3755
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