Graphs | bufferlab

Introductory Activity

The equivalence point occurs halfway up the most vertical region of the graph (see the dotted lines on the left) when 12.0 mL of NaOH are added to the Gatorade. The half-equivalence point occurs when half of that, or 6.0 mL, of NaOH are added. The pH of the equivalence point is 4.00. The pKa of citric acid is be 4.00 because pH=pKa at the half-equivalence point, and the effective buffering range of citric acid must be from a pH of 3.00 to 5.00 because the effective buffering range is from (4.00 - 1.00) to (4.00 + 1.00).

Guided Inquiry

The equivalence point occurs at the halfway point of the vertical region of the graph, in this case when 6.5 mL of NaOH are added to the Gatorade. Thus the half-equivalence point occurs when 3.25 mL of NaOH are added, at a pH of approximately 5.00. The pKa of the weak acid in Gatorade must be 5.00 since pH=pKa at the half-equivalence point, and the effective buffering range of Gatorade must be from a pH of 4.00 to 6.00 because the effective buffering range is from (5.00 - 1.00) to (5.00 + 1.00).

The equivalence point occurs in the most vertical region of the graph, when 16.0 mL of NaOH have been added to the tonic water. Therefore, the half-equivalence point occurs when 8.0 mL of NaOH are added, at a pH of approximately 4.50. The pKa of the weak acid in the tonic water is 5.05 because pH=pKa at the half-equivalence point, and the effective buffering range of tonic water must be from a pH of 4.05 to 6.05 because the effective buffering range is from (5.05 - 1.00) to (5.05 + 1.00).