PERCENT POTASSIUM HYDROGEN PHTHALATE IN AN UNKNOWN SAMPLE
The purpose of this experiment is to determine the mass percent potassium hydrogen phthalate in an
unknown sample, using neutralization titration. The reaction is
KHC8H4O4 (s) + NaOH (aq)  H2O (l) + Na1+ (aq) + K1+ (aq) + C8H4O42– (aq)
The indicator for the endpoint is the familiar phenolphthalein indicator, which turns pink in basic solution.
This experiment is a test of your titration technique, so half of your grade is determined by your accuracy
and precision. You and your partner may standardize the NaOH together, but will have individual
unknowns.
Procedure
1. Preparation of the NaOH solution (do before coming to evening lab)
CAUTION! Sodium hydroxide is extremely corrosive. You must wear goggles, and never
handle the NaOH pellets with your fingers. Use tweezers to pick up any dropped pellets and
dispose of them down the drain. If you get it on your skin, immediately rinse with lots and
lots and lots of cold water.
Prepare 500 mL of 0.1 M NaOH : calculate the required mass of NaOH and weigh it out in a small
weighing cup. Don’t worry about getting the exact mass; you will determine its concentration later.
Put 500 mL of deionized water into a clean 600 mL beaker and add the pellets. Stir thoroughly (use a
magnetic stirrer) until the pellets are dissolved. Store your solution in a clean, labeled bottle.
2. Standardization of NaOH solution.
Fill a clean dry buret with your NaOH solution (if the buret is not dry, consult the instructor).
Note: never, never, NEVER pour above your head!! Take down the buret and hold it below
face level. Be sure the stopcock is closed before you fill it (!).
Run some NaOH solution into a waste beaker to remove air bubbles from the buret tip.
Weigh some dry KHC8H4O4 (about 0.2 g) into a clean 150 mL beaker and record the exact mass. Add
about 30 mL deionized water, 3-5 drops of phenolphthalein solution, and a clean stir bar. Stir
constantly on the stir plate as you titrate carefully to the pale pink endpoint. The solid will dissolve as
you titrate, so be sure all of it is dissolved and none is clinging to the sides of the flask (use your wash
bottle to rinse it) before you accept the endpoint.
Calculate the concentration of the NaOH solution from the balanced chemical equation. Rinse the
beaker and repeat the titration until you are satisfied with the precision of the result (3 sigfigs). This
will require at least two trials and possibly more.
3. Analysis of the unknown sample.
Use the same procedure as above, using about 0.2 – 0.4 grams of unknown sample.
Calculate the mass % KHC8H4O4 in the unknown, and repeat the procedure until you are satisfied
with your precision.
Discussion
Report the % KHC8H4O4 in the unknown. Discuss any problems in the procedure and any data omitted
from the final result.