Data Table 1: Molarity of Unknown Acid Data Table 2: pH\'s for the 3 Titrations
ID: 947630 • Letter: D
Question
Data Table 1: Molarity of Unknown Acid
Data Table 2: pH's for the 3 Titrations
Data Table 3: PKa's and Ka's
Average Ka:
Standard Deviation:
Average Ka:
Standard Deviation:
If the pH at one-half the first and second equivalence points of a diprotic acid is 3.5 and 6.2, respectively, what are the values for pKa1 and pKa2 and Ka1 and Ka2?
Titration 1 Titration 2 Titration 3 Volume of Unknown Acid 13 mL 13 mL 13 mL Molarity of NaOH 0.1 M 0.1 M 0.1 M mL of NaOH at eq pt 1 13 mL 13 mL 14 mL mL of NaOH at eq pt 2 18 mL 17 mL 15 mL Molarity of Unknown .1538 .1385 .1231Explanation / Answer
The Henderson-Haselbalch equation is
pH = pKa + log [base]/[acid]
where "base" and "acid" are the weak acid and its conjugate base.
At half equivalence point, the concentrations of the weak acid and its conjugate base are equal. So that ratio is 1. The log of 1 is zero, so, the pH = pKa.
Therefore pKa1 = 3.5 hence Ka1 = 3.16 x 10-4
andd pKa2 = 6.2 and so Ka2 = 6.31 x 10-7
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