In the first step, a standardized HCl solution is used to titrate the mixture in

Question

In the first step, a standardized HCl solution is used to titrate the mixture in the
                    presence of a pH indicator, phenolphthalein, which has an end point pH of about 8. Once
                    the solution reaches this pH all of the sodium carbonate, but none of the bicarbonate, will
                    have reacted with the acid. Therefore, the number of moles of sodium carbonate can be
                    easily calculated knowing the stoichiometry of the reaction and the volume of acid used
                    in the titration.
                    The second step entails continuing the titration of the mixture resulting from step
                    1. Methyl orange, which has an endpoint at about pH 5, will be used as the pH indicator
                    to monitor the protonation of bicarbonate. The bicarbonate in the mixture reacts with
                    hydrochloric acid forming carbonic acid (equation 6) which then decomposes to form
                    carbon dioxide and water (equation 7). Release of CO2 gas into the atmosphere makes
                    this reaction irreversible
                    
                    Question:
                    
                    4. If the pH indicator used for the titration in step 1 changed color at a lower pH than
                    phenolphthalein, what effect would it have on the mass and wt % of the sodium
                    carbonate determined in this experiment? Explain                 

Explanation / Answer

Procedure

Step 1

1. Rinse the buret with water then with about 5 mL of the HCl standard solution.

2. Fill the buret with standardized 0.1 M HCl solution. Record the exact molarity of the acid as noted on the bottle. Open the stopcock and fill the tip of the buret.

3. Obtain an unknown sample, record its identification code. Weigh out 0.20g (to +/- 0.01 g) of the unknown sample and transfer it to a 125 ml Erlenmeyer flask. Add about 15 ml of distilled water and swirl to dissolve the sample. At this point the solution must be colorless. If the sample does not dissolve completely, don't worry, it will when you start the titration.

4. Add two (2) drops of phenolphthalein indicator. The solution will turn pink.

5. Record the initial volume of acid in the buret. Titrate with the standard HCl until the solution just turns colorless. It is extremely important not to over titrate! If you do over titrate, make sure that a note about it is written in your observations. This solution will be used in step 2, do not discard it!

6. Record the final volume of the HCl solution in the buret.

Step 2

7. Add 2 drops of methyl orange indicator to the colorless solution resulting from the previous titration. The solution will turn yellow.

8. Record the initial volume of acid in the buret. Titrate with the standard HCl until the solution turns light salmon pink (or light peach pink). If the color is red you have over titrated.

9. Record the final volume of the HCl solution in the buret.

10. Dispose of the solution in the flask and clean the flask, rinsing very well with tap water, then distilled water.

11. Repeat Step 1 and Step 2 two more times each using a new 0.20 g sample of the same unknown each time. Buret Cleanup Drain the buret, put any remaining HCl solution into the waste container provided. Leave the buret secured to the ring stand. Close the stopcock and fill the buret with tap water. Drain the buret through the tip. Repeat the rinse once with tap water then once with distilled water. Return the buret to the instructor

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