Acid Base Titrations

Topics: Acid dissociation constant, Titration, PH Pages: 7 (1386 words) Published: November 5, 2013
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EXPERIMENT 9

Acid-Base Titrations

The pH titration curve, found with a known titrant concentrations (here) in a buret, is a procedure for determining the equivalence point of a solution. The shape of the curve allows for correlations based on volume, molarity, pH, etc. to be inferred (McMurry & Fay,p 602). This experiment used both strong-strong and weak acid-strong base titration. The titration curve is distinct for each, as shown in form HCl1 and AA1.


Introduction
Acid-Base Titrations fall into four categories: strong acid-strong base, weak acid-strong base, weak base-strong acid and polyprotic acid-strong base. For strong-strong category titrations, the curve, when plotted with titrant volume on the x-axis and pH as a function of said volume, “will have gradual increases in pH in regions before and after equivalence, with sharp, near vertical increase at equivalence” (McMurry & Fay,p 604). In the second type of reaction, weak acid-strong base titration, “the base will react with the weak acid and form a solution that contains the weak acid and its conjugate base until the acid is completely gone. In order to solve these weak-strong problems, we use the weak acid's Ka value” (http://www.uiowa.edu). Some common formulas and the reaction equations are shown below. Kb = Kw / Ka

M1V1 = M2V2
HCL (aq) + NaOH (aq)  NaCl (aq) + H2O (l)
CH3COOH (aq) + NaOH (aq)  NaCH3COO (aq) + H2O (l)

The NaOH will be the limiting reactant each time, also called the titrant. For each of the acid reactants (both HCl and CH3COOH) the reactions will be broken down into a series of steps or mini reactions that represent 1. Before addition of titrant (0), 2. Before equivalence point (1/4-3/4), 3. At equivalence (1), and 4. Beyond (1&1/4). Data/Procedures

Using a preparation of 0.1 M NaOH prepared from 413mL of distilled water and 7.0mL of a stock 6 M NaOH solution. The resulting 420mL of approximately 0.1 M solution was standardized with KHP. The addition of 0.64 and 0.71 grams KHP, respectively, and 100.0mL distilled water to the two clean 250mL Erlenmeyer flasks was massed and recorded. To each flask was added 3 drops phenolphthalein indicator and swirled till all solids dissolved. To these was added the standardized 0.1 M NaOH from buret until the point when the solution just barely turned pink. A minimum of two trials in agreement within +/- 1% agreement in molarity calculations are required before continuing.

Using the laptop computer, Vernier lab pro interface, drop counter, and pH meter, construct the system so that the flow of information from the pH meter and drop counter instantaneously plot titration data onto the graph provided in the Chemistry with Vernier folder, experiment 24b. Ensuring all equipment and cables are working properly, perform a calibration by filling the buret with distilled water and allowing 10.0mL to flow out and record final volume on screen. Click accept, and calibrate the pH sensor similarly, but using a buffer designated for a pH of 4.0. Place probe into solution, label the value as 4.0 and click accept. For a second calibration value, rinse the probe and place in a buffer solution of 10. Select a pH of 10 for its value and accept as well.

Obtaining 10.0mL of the stock 0.20 M HCl in a 250mL beaker, add 40.0mL of water which will provide 50.0mL of 0.04 M acid solution. The buret was conditioned with the prepared .1 M NaOH and then filled. The 250mL flask containing the standardized HCl solution was carefully placed beneath the buret stand onto which the drop counter was attached carefully as to record the exact amount of titrant entering the flask below. As the stopcock was opened to allow a slow steady flow of titrant, the pH meter and thus the Vernier system continually monitored the gradual, then (at equivalence) sharp rise in pH level for solution. After a considerable plateau in pH was reached, the trial was finished, and three reports...

Cited: McMurry, J.E., & Fay, R.C. 2010. General Chemistry, Atoms First. New York: Pearson College Division.
http://www.uiowa.edu. Accessed 11/03/13.
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