dumped in the sink and the flask washed to repeat the instructions for the remaining two columns on Figure 1. For the second part of the experiment titration of a teaspoon of vinegar that was unknown with sodium hydroxide was observed. A vinegar that was not white vinegar like the one previously used would have a distinct concentration. Instructions from the previous section were repeated although a distinct vinegar used, and results were on Figure 2. For the third part of the procedure the titration of vinegar using a PH meter results were given in Figure 3. From the results given the molarity of acetic acid are to be found. The Henderson-Hasselbach equation was used to find the pK of the acid and vinegar. Data Pre-laboratory Question 1. Write the balanced equation for the neutralization of acetic acid by sodium hydroxide. CH 3 COOH + NaOH -> CH 3 COONa + H 2 0 2. The Food and Drug Administration (FDA) mandates that all vinegar sold in the United States must have a minimum concentration of 4% acetic acid by mass. Calculate the minimum molarity of acetic acid in vinegar according to this standard. Assume the density of vinegar is the same as for water. 40 g/60.04 ) x 1 L = o.67 mol/L 3. Calculate the mass of NaOH required to make 0.40 L of a 0.50 M solution. 0.400L * 0.5 M NaOH = 0.2 mol NaOH 0.2 mol NaOH * 39.997 NaOH/1 Mol NaOH = 0.8g NaOH Laboratory Questions
Mejia - 4 - 1. Based on the experimental data, what is the molarity of the vinegar tested? How does this value compare with those determined in the pre-lab questions? The results were a bit lower, but the molarity of the solution could have affected 2. List at least two possible sources of error from the experiment. Would these errors result in an overestimation or an underestimation of the concentration of acetic acid in the vinegar? The NaOH solution could have been added too late or too early to the vinegar solution. The amount of acetic acid may not have been the right amount. Discussion Questions 1. Describe on a molecular level the difference between two titrations—one, of a strong acid with a strong base and the other, of a weak acid with a strong base. Draw a titration curve for each situation to further illustrate the differences between strong and weak acids.
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- Fall '08
- pH, Sodium hydroxide, Marta Mejia