1. The phosphoric acid in a 100.00-mL sample of Cola drink was titrated with 0.1

Question

1. The phosphoric acid in a 100.00-mL sample of Cola drink was titrated with 0.1025 M NaOH. If the first equivalence point occurred after 16.11 mL of base was added, and the second equivalence point occurred after 32.55 mL of base was added, calculate the molar concentrations of H3PO4 in the Cola sample based on

(a) the first equivalence point

(b) the second equivalence point.

2. On the titration curve for question 1, it was determined that at midpoint of one of the buffer regions the pH was 2.15. What are the pKa and Ka of phosphoric acid at this midpoint? Show your work.

Explanation / Answer

1.

Titration of cola sample

H3PO4 in cola sample is titrated with NaOH

a.

First equivalence point

volume of NaOH consumed = 16.11 ml

moles of NaOH = 0.1025 M x 0.01611 L

= 0.00165 mol

moles H3PO4 present = 0.00165 mol

molar concentration of H3PO4 in cola sample = 0.00165 mol/0.1 L

= 0.0165 M

b.

Second equivalence point

volume of NaOH consumed = 32.55 ml

moles of NaOH = 0.1025 M x 0.03255 L

= 0.00334 mol

moles H3PO4 present = 0.00334 mol/2

= 0.00167 mol

molar concentration of H3PO4 in cola sample = 0.00167 mol/0.1 L

= 0.0167 M

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