pH: Lab Report

Before You
Begin: You may either copy and paste this
document into a word processing program of your choice or
print this page.

Procedure:

Part I: Titration with an Indicator

1. Fill the 50-milliliter buret with a 0.25 molar NaOH
   solution.
2. Record volume.
3. Measure out between 20 milliliters and 40 milliliters
   of the unknown HCl solution.
4. Record volume.
5. The amount of unknown HCl is then added to the
   100-milliliter Erlenmeyer flask.
6. Add two drops of the indicator, phenolphthalein, to the
   acid in the flask.
7. Using the slider on the right hand side, add NaOH to
   the HCl in the Erlenmeyer flask (This action is known as
   titrate). Add the indicator until
   the color of the indicator turns a light shade of
   pink.
8. Record the volume.
9. If you went too far past the end point, please record
   the results of your trial. Include all sets of data in your
   lab report.

Part II: Titration with a pH Meter

1. Fill the 50-milliliter buret with a 0.25 molar NaOH
   solution.
2. Record volume.
3. Measure out between 20 milliliters and 40 milliliters
   of the unknown HCl solution. This amount must be
   different than the amount used in part I.
4. Record volume.
5. The amount of unknown HCl is then added to the
   100-milliliter Erlenmeyer flask.
6. Insert the pH meter into the Erlenmeyer flask and
   record the initial pH of the acid. Remember to record the
   pH of the solution after every 5 milliliters of NaOH
   added.
7. Titrate the NaOH into the HCl until the buret is
   empty.

Data and Observations:

Part I: Titration with an Indicator

Insert a complete data table, including appropriate
significant figures and units, in the space below. Be sure to
include all trials that were completed in Part I. Also
include any observations that you made over the course of
Part I.

A data table for recording your observations is
below:

Part II: Titration with a pH Meter

1. Insert a complete data table, including appropriate
   significant figures and units, in the space below. Be
   sure to include all trials that were completed in Part
   II. Also include any observations that you made over the
   course of Part II. A data table for recording your
   observations is below:

   Observations

2. Plot the data from your data table into a graphing
   problem. Include the graph below. (pH should be on the
   y-axis, volume of NaOH added should be on the
   x-axis).

Calculations:

Complete the following calculations for each part of the
lab, showing your work neatly, labeling each calculation with
a subtitle, and giving a brief written explanation of each
calculation.

1. Determine the volume of sodium hydroxide added to the
   flask from the buret.
2. Calculate the molarity of the hydrochloric acid in the
   flask. You may refer to the Titration demo at the beginning
   of the honors lesson, just above the Virtual Lab to sample
   calculations.

Conclusion:

1. Describe the graph of pH values over the course of the
   reaction in Part II. Was the change in pH consistent over
   the course of the reaction? Do your best to explain the
   reason for the shape of the pH curve in your own
   words.
2. Why is it important to stir the solution in the flask
   as you add titrant from the buret?
3. If a student did not remove all of the bubbles from
   inside the buret before reading the initial volume and
   beginning the titration, will this cause the calculated
   concentration of the hydrochloric acid determined from that
   trial to be higher or lower than the actual concentration?
   Explain your answer in complete sentences.
4. Describe at least three possible sources of error in a
   titration experiment like this and, for each, explain
   whether the error would result in a calculated molarity
   that is too high or too low.