Lastly a Second Derivative Plot was graphed which is showed in Figure 4 For

# Lastly a second derivative plot was graphed which is

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Lastly, a Second Derivative Plot was graphed, which is showed in Figure 4. For this graph, one can see the endpoint at which y is equal to zero. 0 5 10 15 20 25 30 -8 -6 -4 -2 0 2 4 6 8 10 V'' dpH2/dV2 Figure 4. Second Derivative Plot 3

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The KHP analyzed in the experiment follows the dissociation reaction: + ¿ ¿ + H ¿ KHP↔KP ¿ In order to calculate the acid-dissociation constant of KHP, the half-equivalence point was determined. At half-equivalence point, where the volume of titrant used is half of the volume used at equivalence point, the number of moles of the KP - is equal to the number of moles of KHP left. Therefore, given the pH at half-equivalence point and using the formula for the acid-dissociation constant, the Ka can be calculated. ¿ KP ¿ ¿ + ¿ H ¿ ¿ ¿ Ka = ¿ At ½ equivalence point, ¿ KP ¿ ¿ ¿ ¿ KP ¿ ¿ + ¿ H ¿ ¿ ¿ Ka = ¿ + ¿ H ¿ → pKa = pH Ka = ¿ The percent purity, on the other hand was calculated by using the formula: %KHP = M NaOH ×V NaOH × 1 mol KHP 1 mol NaOH × 204.2 g mol KHP g KHP sample × 100% It was found out that the sample’s percent purity is 69.24% with a confidence interval of + 0.3 and a pKa of 4.87 with a confidence interval of 4.87 + 0.01. When compared to the book value of KHP’s pKa which is 5.51 at room temperature, the calculated percent error was 11.62%. 4
The two trials yielded slightly different endpoints, hence, also different half-equivalence points and volume used in calculating the acid-dissociation constant. Also, because several very small increments of titrant were added at each point, there were errors due to volume measurements. Sources: Skoog, et al., Fundamentals of Analytical Chemistry, Eighth edition, 2004 Day, Underwood, et al. Quantitative Analysis, 1967 Christian, G.D. Analytical Chemistry, 1986 5
• Fall '17
• pH, potentiometric titration

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