Acid/Base Titration Lab
Background: Titration involves the addition of a solution whose concentration is known to a solution whose
concentration is unknown. The volume of the known solution required to react completely with a known volume of
the solution whose concentration is being determined is measured. An indicator is added to the solution to mark
the point at which the two quantities reach equivalence.
In this experiment sodium hydroxide of known concentration will be added to a known volume of hydrochloric acid
to determine the molarity of the hydrochloric acid. Phenolphthalein will be used as the indicator. Phenolphtalein
is pink in a basic solution and clear in an acidic solution.
Objective: To determine the molarity of an unknown concentration of hydrochloric acid.
Materials:
Ring stand
Buret
Clamp
2 beakers
1 Erlenmeyer flask
DI water
unknown acid
funnel
phenolphthalein dropper
0.1 M NaOH
50 mL graduated cylinder
Procedure:
1. Set up the ring stand, buret, and buret clamp. Make sure that the buret remains vertical at all times.
2. Add some DI water to the buret to check for leaks. If there are no leaks, drain the DI water completely from
the buret into the smaller beaker.
3. Fill your larger beaker with NaOH from the stock supply. Carefully add the NaOH to the buret. You will need
to drain some of the NaOH from the buret to fill the tip. Fill the buret so that the meniscus of the NaOH is
sitting on the 0.00 mL mark with the tip filled.
4. Use the graduated cylinder to add exactly 40 mL of the hydrochloric acid to the Erlenmeyer flask.
5. Add 2 – 3 drops of phenolphthalein to the acid solution in the Erlenmeyer flask.
6. Place the Erlenmeyer flask under the buret. Place a sheet of white paper under the Erlenmeyer flask.
7. You are now ready to begin titrating. Carefully add about 1 mL of the NaOH from the buret to the Erlenmeyer
flask and swirl to mix. Continue adding 1 mL increments until you begin to see a pink color. The pink color
should appear briefly and then quickly disappear.
8. Once you have seen the pink color, begin adding the NaOH slowly and in smaller amounts. Remember to
swirl the flask after each addition of NaOH. As you near the endpoint, the pink color should stay longer, but
will still disappear when the flask is swirled. The longer the pink color stays, the smaller your next increment
of NaOH should be. You want to eventually be adding the NaOH one drop at a time until the palest pink
color appears and stays.
9. When the pale pink color appears and stays you have reached the endpoint. Record the buret reading to the
nearest 0.01 mL.
10. Dump out the Erlenmeyer flask contents into the red bucket and rinse with your DI water. With your clean
Erlenmeyer flask repeat steps 3 – 9 twice more (the Erlenmeyer does not need to be dry, only clean). You
will need to add more NaOH to the buret for the second and third trials.
11. Clean out the Erlenmeyer and dispose of any remaining acid or base according to your teachers instructions.
Data Table:
Trial 1
Final Buret Reading (mL)
Initial Buret Reading (mL)
Volume of Base (mL)
Molarity of Base (M)
Volume of Acid (mL)
Molarity of Acid (M)
Average Molarity of Acid (M)
Trial 2
Trial 3
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Processing the Data:
1. Write a balanced equation for the reaction in this titration.
2.
Calculate the molarity of the acid. Show all work.
3. Calculate the average molarity of the acid. (No, you don’t have to show work here but make sure I can find
it!)
Postlab questions:
1. Describe the apparent relationship between [H3O+1] and [OH-1] when the endpoint is reached in an acid-base
titration.
2. If 30.0 mL of 0.500 M KOH are needed to neutralize 10.0 mL of HCl of unknown concentration, what is
molarity of the HCl?
3. How many mL of 0.100 M NaOH are needed to titrate 20.0 mL of 0.300 M H2SO4?