Lab 4.2 Acid-Base Titrations
Introduction
The molarity of an acid can be calculated by accurately neutralizing it with a base of known molarity. As acid is gradually added
to a measured volume of base, an indicator will cause a color change that indicates the end point is reached. Knowing the
measured volume of the acid as well, the molarity of the acid can be found by the equation:
_____ Liters(base) | _____ moles(base) | _____ moles(acid) |
=
|
1 Literbase) | _____ moles(base) | _____ Litersacid)
In this experiment you will neutralize sodium hydroxide with hydrochloric acid, using phenolphthalein as the indicator, which
will cause the solution to turn from red (basic solution) to clear (acidic solution). You will know the end point is reached when
the color of the solution is a pale pink color, almost clear.
Doing at least three trials will help you obtain a more accurate value for the molarity of your solution, which of course will be
graded as an unknown, to be turned in to the teacher at the end of the period.
Procedure:
1. Clean burets as instructed by your teacher. Label the clamp as HCl on one side, and NaOH on the other, as instructed. Label
the beakers as well. The smallest beaker label HCl. The middle size beaker label NaOH, and the largest beaker label Waste.
Fill the smallest beaker with 40mLs of HCl and the middle size beaker with 40mLs of NaOH.
2. Fill your burets with the NaOH and the HCl assigned to your lab group. Fill close to the ‘0ml’ line, but it doesn’t have to be
exactly ‘0mL’. Don’t go above ‘0mL’. If you accidentally do, just drain out a little solution into your waste beaker.
CAUTION! HCl AND NaOH ARE CORROSIVE! WEAR SAFETY GLASSES AND WASH HANDS!
3. Record the initial HCl and NaOH levels as read on your burets in the data table, under initial volume. Read to the nearest 0.01mL.
4. Drain out roughly 10mLs of NaOH into your clean flask. Don’t record the final volume level yet.
5. Add phenolphthalein to your flask.
6. Titrate out HCl until you reach the equivalence point. This may not occur until after quite a bit of acid has been drained into your
flask. But when it occurs it will be a very quick change from red to clear. You may need to go back and add more NaOH if you
overshoot the equivalency point. Work both solutions until you achieve the pale pink color.
7. Record the final volumes of your burets in the data table, to the nearest 0.01mL.
8. Dump the flask into the waste beaker, clean it, and fill the burets back to the top, and you are ready to begin trial 2.
9. Perform three accurate titrations, cleaning your flask before each trial.
10. The volume of each substance used in each trial can be found by subtracting Final Volume – Initial Volume
11. Clean your lab station after your three trials. You may leave the solutions in the burets.
12. You must turn in a piece of paper with all your lab partners’ names, your acid letter, and its molarity by the end of the period.
It should look like the sample on the far right:
Data Table
NaOH
Trial 1
Molarity
0.100M
HCl
Molarity
NaOH
Trial 2
Molarity
0.100M
HCl
Molarity
NaOH
Trial 3
Molarity
0.100M
HCl
Molarity
Initial Volume
Final Volume
Volume Used
Initial Volume
Final Volume
Volume Used
Initial Volume
Final Volume
Volume Used
Initial Volume
Final Volume
Volume Used
Initial Volume
Final Volume
Volume Used
Initial Volume
Final Volume
Volume Used
Sample Separate Paper due @ end of period
Name 1
Name 2
Name 3
Name 4
Molarity of HCl
for lab group # _____
is:
______ M
Calculations
1. Show your three calculations for the molarity of the acid on the back of this write-up. Average your data to come up with an accurate
measure of the molarity of the acid. (You may throw out any poor measurements, but please indicate.)
2. The following day, your teacher will give you the exact molarity of your acid. Please calculate the percent error, using the equation:
| Exact molarity – experimental molarity | X 100%
Exact molarity
Questions
1. Describe the apparent relationship between H+ and OH- concentrations when an end point is reached in an acid-base titration.
2. The indicated end point of an acid-base titration seldom occurs at a pH of 7. What determines the pH of the end point?
(Hint: For example, we used phenolphthalein in this lab. Look it up in your textbook or online. At what pH does its color change?)
Further Investigations
1. Government agencies are continually monitoring the production and advertising of consumer products. Select some common
foods, medications, and cleansing solutions and determine the pH of each.
Lab 4.2 Acid-Base Titrations
Name ______________________
Pre-lab questions: The following data was obtained from a regular chemistry lab group conducting this lab:
NaOH
Initial Volume Final Volume
Volume Used
Trial 1
Molarity
0.100M
1mL
12mL
HCl
Initial Volume Final Volume
Volume Used
Molarity
2mL
9mL
1. Fill in the rest of the data table.
2. Show your calculation for the Molarity of the HCl solution. If you used a calculator (hopefully), leave the answer on the screen.
3. Knowing my regular chem classes, this trial may have lacked accuracy, but it definitely lacked precision.
Please list the obvious non-precise measurements.
4. Look at your calculator answer. Round the answer to the correct number of significant digits.
If you look at the first 3 digits on your calculator screen, how different do your 2 answers look?
5. If the actual molarity of HCl was 0.150M, please find the % error for the answer with the correct sig digs.
6. Can you see how precise measuring can be as important as accurate measuring?
Objectives:
Procedure:
Data Table
NaOH
Trial 1
Molarity
0.100M
HCl
Molarity
NaOH
Trial 2
Molarity
0.100M
HCl
Molarity
NaOH
Trial 3
Molarity
0.100M
HCl
Molarity
Calculations:
1. Trial 1:
Initial Volume
Final Volume
Volume Used
Initial Volume
Final Volume
Volume Used
Initial Volume
Final Volume
Volume Used
Initial Volume
Final Volume
Volume Used
Initial Volume
Final Volume
Volume Used
Initial Volume
Final Volume
Volume Used
Trial 2:
Trial 3:
2. Percent error
Questions
1. Describe the apparent relationship between H+ and OH- concentrations when an end point is reached in an acid-base titration.
2. The indicated end point of an acid-base titration seldom occurs at a pH of 7. What determines the pH of the end point?
(Hint: For example, we used phenolphthalein in this lab. Look it up in your textbook or online. At what pH does its color change?)
Further Investigations
1. Government agencies are continually monitoring the production and advertising of consumer products. Select some common
foods, medications, and cleansing solutions and determine the pH of each.