304 ❯ STEP 4. Review the Knowledge You Need to Score High
❯ Free-Response Question
You have 15 minutes to answer the following question. You may use a calculator and the
tables in the back of the book.Question
A sample of a solid weak monoprotic acid, HA, is supplied, along with solid sodium
hydroxide, NaOH, a phenolphthalein solution, and primary standard potassium hydrogen
phthalate (KHP).
(a) Describe how a standardized sodium hydroxide solution may be prepared for the
titration.
(b) Sketch a graph of pH versus volume of base added for the titration.
(c) Sketch the titration curve if the unknown acid were really a diprotic acid.
(d) Describe the steps to determine Ka for HA.
(e) What factor determines which indicator should be chosen for this titration?❯ Answer and Explanation
(a) A sample of sodium hydroxide is weighed and dissolved in deionized water to give
a solution of the approximate concentration desired. (Alternatively, a concentrated
NaOH solution with unknown concentration could be diluted.) The process is as
follows:- Weigh samples of dried KHP into flasks and dissolve in deionized water.
- Add a few drops of the appropriate acid–base indicator (phenolphthalein) to each
sample. - Rinse a buret with a little of the NaOH solution; then fill the buret with the NaOH
solution. - Take the initial buret reading.
- Titrate the NaOH solution into the KHP samples until the first permanent pink
color appears. - Take the final buret reading.
- Using the molar mass of KHP, determine the moles of KHP present. This is equal
to the moles of NaOH. - The difference in the buret readings is the volume of NaOH solution added
(convert this to liters). - The molarity of the NaOH solution is the moles of NaOH divided by the liters of
NaOH solution added. - (Repeat the procedure for each sample.)
Give yourself 3 points for this full list, if the items are in order. If three to five items
are in the wrong order or missing, you get only 2 points. If five to seven items are in the
wrong order or missing, you get only 1 point. You get 0 points for three or fewer items.