This is an example of an error analysis–type item.

  1. A student titrated 10.00 mL of fruit juice with 12.84 mL of 9.580 x 10-2 M NaOH. Which step is NOT correct in this calculation of mass of citric acid in 1.00 mL of juice?

    C3H5O(COOH)3 + 3 NaOH → Na3(C3H5O(COO)3 + 3 H2O

  1. Moles NaOH = (12.84 mL) (9.580 x 10-2 mol/L).

  2. Moles citric acid = (1.230 mol NaOH) (1 mol citric acid / 3 mol NaOH).

  3. Mass citric acid in sample = (0.4100 mol citric acid) (192.12 g/mol citric acid).

  4. Mass citric acid in 1 mL fruit juice = (78.77 g citric acid) / (10.00 mL fruit juice).

  5. All the steps are correct.

    This is an example of a laboratory-related item.

  1. Consider a laboratory exercise in which you prepare approximately 200 mL of a pH = 4.35 buffer. The chemicals available for your use are: 1M solutions of acetic acid, ammonium acetate, ammonium chloride, aqueous ammonia, hydrochloric acid, sodium acetate, and sodium hydroxide (acetic acid Ka = 1.8 x 10-5 and ammonia Kb = 1.8 x 10-5).

  1. Which chemicals will you use to make your buffer? Explain your choice; use chemical equations where appropriate.

  2. How much of each chemical will you need? Show calculations to support your answer.

  3. Outline the procedure you will follow to make your buffer. Include specific equipment and glassware you will use.

  4. How will you prove that you successfully prepared the buffer?


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