# Chemistry Lab

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A 15.00 mL sample of a weak acid with Ka = 3.52x10-4, was titrated with 0.475 M KOH. The equivalence point was reached after addition of 19.5 mL of KOH with the phenolphthalein indicator. Determine the molar concentration of the original acid solution, then find pH and a % ionization of the acid in the original solution.

How do I set up this problem? I don't know where to start.

• Chemistry Lab -

HA + NaOH ==> NaA + H2O
mols NaOH = M x L = ?
mols HA = mols NaOH (look at the equation. 1 mol HA = 1 mol NaOH)
M HA = mols HA/L HA.

.........HA ==> H^+ + A^-
I........?M......0......0
C.........-x.....x.......x
E........?M -x...x.......x

Ka = (H^+)(A^-)/(HA)
Substitute and solve for (H^+) and convert to pH. pH = -log(H^+).

Finally, %ion = [(H^+)/M]*100 = xx

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