Chem100A_Exp3_Mixture of CO3&OH & Citric Acid in Soft Drink_Quiz_20110411_KEY

Chem100A_Exp3_Mixture of CO3&OH & Citric Acid in Soft Drink_Quiz_20110411_KEY

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Spring 2011 Student ID:_______________________________ Experiment 3: Mixture of Carbonate and Hydroxide and Citric Acid in Soft Drink Quiz #2 (20 points)–April 11, 2011 1. (4 points) Explain the difference between the endpoint and the equivalence point in a titration. The equivalence point is when the moles of titrant are stoichiometrically balanced with those of the analyte. The endpoint is simply the point at which the indicator changes color. The difference between the two is usually very small due to the logarithmic pH scale (if the indicator has been properly chosen). 2. (4 points) What is the purpose of boiling the deionized water in this experiment? (mention in relation to LeChatlier’s principle) Boiling removes dissolved CO 2 . By LeChatlier’s principle, this drives the following reaction to the right and reduces the concentration of carbonic acid. This prevents the extra
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Unformatted text preview: acidity from alerting the titration endpoint. H 2 CO 3 (aq) CO 2 (aq) + H 2 O 3. (4 points) Give the pH range for the following indictor transitions: a. Phenolphthalein transitions from colorless to fuschia: Clear at 8.2 to fuschia at 10 b. Bromocresol green transitions from blue to yellow: Yellow at 3.8 to blue 5.4 4. (4 points) Why do we have to perform two titrations using the indicators above to analyze our unknown sample? Spring 2011 Student ID:_______________________________ 5. (4 points) Sketch a titration curve for Na 2 CO 3 vs HCl. Label the axes and label the equivalence points and indicator location. Please take a look at the curve titled Na 2 CO 3 and NaOH vs. HCl in your expt 2 lecture notes....
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Chem100A_Exp3_Mixture of CO3&OH & Citric Acid in Soft Drink_Quiz_20110411_KEY

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