Experimental Procedure

A Preparation of the 0.10 M NaOH
1. Use a 10 mL graduated cylinder to measure out ~ 8 mL of 2.5 M NaOH.
2. Add this to a 200 mL volumetric flask.
3. Fill the flask to the line with deionized water.
4. Cork the flask and carefully invert it several times to make sure the solution is homogenous.

B. Standardization of the 0.10 NaOH Solution. NaOH is hydroscopic and absorb water from the air causing the concentration of any sodium hydroxide solution to change over time. It is necessary to calculate the exact concentration of NaOH.

1. Label two 50 mL flasks for Trial 1 and Trial 2. Measure ~0.3 (+/- 0.05) g of KHP for each trial.
2. Record the exact mass in your data section.
3. Dissolve the KHP by adding ~ 15 mL water to each flask. You may need to swirl for several minutes to dissolve.
4. If there is KHP on the sides of the flask use a water bottle to help dissolve the solid.
5. Add 3-4 drops of phenolphthalein indicator to each flask.
6. Check your buret to make sure it will not leak by adding deionized water to the top and allowing it to run out as shown by your instructor.
7. Carefully add ~ 50 mL of the ~ 0.10 M NaOH solution to a buret. Do not waste time getting it to the 0.0 mark. Instead record the exact volume (to two decimal spaces) in your data section.
8. Titrate the sample slowly until a pale pink color remains for at least 3 seconds. If you exceed the endpoint (even by a single drop) the color will be a dark pink and you may have to do a third trial.
9. Obtain instructor approval on the color of the solution before moving on.
10. Record the final volume of NaOH in the data section. You do not need to add more NaOH to theburet between trials. Use the final volume from the first trial as the initial volume for the second trial.
11. Use the volume of NaOH and grams of KHP to calculate the molarity of the NaOH solution.
12. Record the concentration of your standardized NaOH solution in your data section. You will usethis molarity in all subsequent measurements.

C. Preparation of the Antacid Sample

1. Label 2 50 mL Erlenmeyer Trial 1 and Trial 2.
2. For each trial, measure ~0.2 g of your antacid (is your sample is a solid) or ~1.0 g of antacid (if your sample is a liquid) into a 50 mL Erlenmeyer flask.
3. Measure 25 mL of HCl. Record the exact volume and carefully add it to the antacid sample.
4. Obtain the exact concentration of HCl used from your instructor. Record in your data section.
5. Gently swirl to help the antacid dissolve.
6. Add 3-4 drops Phenolphthalein to both flasks.
7. Heat the samples to almost boiling for 5 minutes to remove all dissolved carbon dioxide.
8. Allow to cool for ~10 minutes prior to continuing the procedure.

D. Back Titration of the Antacid Sample
1. Fill your buret with ~ 50 mL standardized NaOH.
2. Titrate the sample slowly until the entire solution is a pale pink color. The color should remain for at least 3 seconds. If you exceed the endpoint you may have to do a third trial.
3. Obtain instructor approval on the color of the solution before cleaning up.

Pre-lab Assignment/Questions

N o t e – this pre-lab must be finished before you come to lab. (Please see syllabus for how to submit this assignment.)

1. Write the balanced equation for the reaction of the active ingredient in Phillips’ Magnesia with HCl.
2. Calculate the volume of 2.5 M NaOH needed to make 200 mL of 0.1 M NaOH.
3. A 0.315 g sample of Tums (containing CO32-) is dissolved in 25.0 mL of 0.102 M HCl. The hydrochloric acid that is not neutralized by the Tums is back titrated with 8.75 mL of 0.102 M NaOH.a. How many mols of base are in the antacid?

b. Calculate the neutralizing capacity (in mol acid / g antacid) of Tums according to these results.

Experimental Data and Results

*Include all work, units and write answers in scientific notation (if applicable) using the correct number of significant figures for full credit. Obtain a signature prior to leaving lab.
A Preparation of the 0.10 M NaOH

B. Standardization of the 0.10 NaOH Solution

D. Back Titration of the Antacid Sample

Check this box if all materials in your tray have been cleaned, the trays and other materials have been returned to their proper position, and all items in your drawer are accounted for.

Instructor Signature_______________________________________________________________

Post Lab Questions

*See this syllabus for instructions on how to turn in this section of the lab handout.

1. AstudentwhoisstandardizingtheNaOHsolutionfillstheburetwiththe NaOH solution but does not open the stopcock to fill the tip of the buret before starting the titration. Will the NaOH concentration determined from this titration be higher or lower than the actual concentration? Explain.
2. List possible sources of error in this experiment and for each, indicate whether the effect would be an erroneously high or low value for the neutralization capacity of the antacid. Use errors different than the one listed in question 1. For full credit post at least 2 errors.
3. The HCl concentration in a hyperacidic stomach is 0.03 M. The volume of liquid in the stomach is 300 mL. If a tablet contains approximately 2 grams of antacid per tablet, then how many tablets of the antacid would have to be taken to bring the concentration of HCl in the stomach to the more normal level of 0.0003 M? Show your work.