20/02/2020
Titration Lab
Sorrawit Chalermnai (Mark) 6161024
Pichitphon Kempetch (Porsche) 6161029
Jirapol Worrakitkanwasin (Kim) 6161140
Summary/abstract
In this lab, we are trying to find the concentration of an unknown acid solution. The process
used to determine the concentration is called titration. The results were very close. All in all, we were
able to indicate the concentration of an unknown acid solution.
Introduction
Acid and base are terms that we all had heard in our daily life. Casually, they are being used
in industrial or biological needs. The Arrhenius definition states that an acid produces H+ in solution
−
and a base produces 𝑂𝐻 (Clark, 2019). Sometimes in a Chemistry lab, it is essential to know the
concentration of acid and base. The process to determine this is called acid-base titration. The
procedure is done by adding a standard solution of acid or base having known concentration to an
unknown solution until it is completely neutralized. Significantly, a pH indicator is required to
monitor the progress of the acid-base reaction depending on the color scale of a specific indicator.
Then, you would have to record the volume of the standard solution you added to the unknown
solution in order to evaluate its concentration. The formula that is used to find the concentration is a
𝑀𝑎𝑉𝑎 = b𝑀𝑏𝑉𝑏 where M is the concentration and V is the volume.
Objective
To identify the proton concentration of acid using a titration process.
Materials
1. 100 mL of Sodium Hydroxide
2. Phenolphthalein Solution
3. Buret
4. Funnel
5. The Volumetric Pipette
6. Erlenmeyer Flask
7. Ring Stand and clamp
8. Waste Beaker
9. Bromothymol blue
Method
1. Record the molarity of the sodium hydroxide solution on the datasheet
2. Obtain about 100 mL of the sodium hydroxide solution in a clean beaker. This should be enough
for the initial cleaning of your buret and for your first 3 trials.
3. Clean your buret: Add about 5 mL of the base solution from the beaker to the buret
(use a funnel to pour). Move the funnel around while adding to ensure the sides of the buret are
coated with base. Alternatively, you can remove the buret with the 5 mL of titrant from the buret
stand and carefully tilt and rotate to coat all interior surfaces with the titrant. Drain the solution
through the stopcock into a waste beaker. Repeat this rinse with a second 5 mL portion of base.
4. Pour more of the sodium hydroxide solution into the buret un.l it is near the 0.00 mL mark. Open
the stopcock to allow several drops to rinse through the tip of the buret. This should eliminate any air
bubbles in the buret tip. Record your initial buret reading on the datasheet for trial 1 (the volume
does not need to be exactly 0.00 mL).
5. Draw 10.00 mL of the acid solution into the volumetric pipette and transfer this solution into an
Erlenmeyer flask. Add 2-3 drops of phenolphthalein to the acid solution in the flask.
6. Place the flask under the buret and start adding the base solu.on to the Erlenmeyer flask. Have
one lab partner swirl the flask while the other controls the stopcock. When pink starts to develop,
add the solution more slowly. At this point, you should add one drop at a time followed by swirling
until a very light pink color persists for at least 30 seconds. Remember, the lighter the pink the
beaker!!!
7. Record the final reading of the buret. Wash the contents of the flask down the drain with water.
8. Refill the buret with more sodium hydroxide solution if necessary. Record the new volume under
trial 2 on the datasheet. Pipette another sample of acid and add the phenolphthalein as before and
titrate as before.
9. Conduct additional concentrations until two of them differ by no more than 1.0%.
10. Repeat step 5 by using Bromothymol blue as an indicator instead of phenolphthalein.
Results
Phenolphthalein Bromothymol blue
Trial 1 Trial 2 Trial 1 Trial 2
Initial buret 81.30 67.95 52.45 98.5
volume (mL)
Final buret 65.65 52.2 36.40 82.50
volume (mL)
Volume of base 15.65 15.75 16.05 16
(mL)
Volume of base 0.01565 0.01575 0.01605 0.016
(L)
Moles of base −4 −4 −4 −4
7. 825 × 10 7. 875 × 10 8. 025 × 10 8 × 10
(mol)
Acid to Base 1:1 1:1 1:1 1:1
Mole Ratio
Moles of acid −4 −4 −4 −4
7. 825 × 10 7. 875 × 10 8. 025 × 10 8 × 10
(mol)
Volume of acid 0.01 0.01 0.01 0.01
(L)
Acid 0.07825 0.07875 0.08025 0.08
concentration
(M)
Average 0.0785 0.080125
concentration
(M)
pH 1.105 1.09
Calculations
Phenolphthalein:
Trial 1
𝑀𝑉𝑎 = 𝑀𝑏𝑉𝑏
0.05 x 15.65 = X x 10
0.7825 = 10 X
X = 0.07825 Mol
Trial 2
𝑀𝑎𝑉𝑎 = 𝑀𝑏𝑉𝑏
0.05 x 15.75 = X x 10
0.7875 = 10 X
X = 0.07875 Mol
average
0.07825+0.07875
2
= 0.0785
pH
+
pH = - log [𝐻 ]
pH = - log [0.0785]
pH = 1.105
Bromothymol blue:
Trial 1
𝑀𝑎𝑉𝑎 = 𝑀𝑏𝑉𝑏
0.05 x 16.05 = X x 10
0.825 = 10 X
X = 0.0825 Mol
Trial 2
𝑀𝑎𝑉𝑎 = 𝑀𝑏𝑉𝑏
0.05 x 16 = X x 10
0.8 = 10 X
X = 0.08 Mol
average
0.0825+0.08
2
= 0.08125
pH
+
pH = - log [𝐻 ]
pH = - log [0. 08125]
pH = 1.09
Discussion
𝑁𝑎𝑂𝐻 + 𝐻𝐶𝑙 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂
According to the experiment, the concentration of basic solution was given with 0.05
molarity and asked for the acid one by using the titration method to identify the equivalence point
and end point. In addition, NaOH is the strong base while the HCl is the strong acid. As we dropped
the base solution into acid with buret, the color of the solution turns into various kinds of colors as
the pH changes. When it reaches the endpoint, the color of the solution will remain unchanged.
The first round of Phenolphthalein was used 15.65mL of NaOH and the color changed into
light pink. Moreover, the second trial of the same indicator was reduced 15.75mL NaOH, offering the
same color as previous round.
In addition, we changed the indicator into Bromothymol blue and repeated the same steps
as phenolphthalein. It used 16.05mL of NaOH and changed the color to green at the equivalence
point for the first trial of Bromothymol blue. 16mL was used for the second round and had the same
color as the first trial. When both of them reached the endpoints, they turned blue and remained
unchanged. The average concentration of Phenolphthalein is 0.0785 while the average of
Bromothymol blue was 0.080125. Furthermore, the pH of Phenolphthalein and Bromothymol blue
were 1.105 and 1.09, respectively.
In the experiment, there were some human errors that occurred. One of which would be
dropping too much base at once and the solution reached the endpoint. This means we had to do
some rounds extra for accuracy and more precise. Another obstacle would be reading the wrong
scale of buret and it provided the wrong amount that was used.
Conclusion
In conclusion, the results from two indicators were very close. The thing
that we learn from this experiment is to calculate the unknown concentration of acid from base
concentration. Moreover, finding the results from the amount of HCl that is given.
References
Libretexts. (2019, September 30). Overview of Acids and Bases. Retrieved from
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supp
lemental_Modules_(Physical_and_Theoretical_Chemistry)/Acids_and_Bases/Acid/Overview_of_Acid
s_and_Bases
Bell, R. P. (2018, November 27). Acid–base reaction. Retrieved from
https://www.britannica.com/science/acid-base-reaction
Post-lab Questions
1.How would it affect your results if you used a beaker with residual water in it to measure out your
standardized sodium hydroxide solution?
- It could lower down the alkalinity of the sodium hydroxide solution causing an error in the
titration process later on the experiment.
2.How would it affect your results if you used a wet Erlenmeyer flask instead of a dry one when
transferring your acid solution from the volumetric pipette?
- It could increase the pH of the acid solution causing an error in the titration process later on
the experiment.
3. How do you tell if you have exceeded the equivalence point in your titration?
- Each indicator would have a color scale to indicate whether it is already neutralized or not. If
it exceeded the equivalence point, it would turn into the color on the scale where the titrant
moles is more than the moles the substance being titrated.
4.Vinegar is a solution of acetic acid (CH3COOH) in water. For quality control purposes, it can be
titrated using sodium hydroxide to assure a specific % composition. If 25.00mL of acetic acid is
titrated with 9.08 mL of a standardized 2.293 M sodium hydroxide solution, what is the molarity of
the vinegar?
- 𝐶𝐻3𝐶𝑂𝑂𝐻(𝑎𝑞) + 𝑁𝑎𝑂𝐻(𝑎𝑞) → 𝐻2𝑂(𝑙) + 𝐶𝐻3𝐶𝑂𝑂𝑁𝑎(𝑎𝑞)
molarity : NaOH(aq) = (concentration NaOH)(volume of NaOH)
0. 0098 × 2. 293
0. 0208 𝑚𝑜𝑙
0.0208 molarity NaOH = 0.0208 M of 𝐶𝐻3𝐶𝑂𝑂𝐻
0.0208
𝐶𝐻3𝐶𝑂𝑂𝐻concentration = 0.025
=0.832 mol/L
vinegar molarity = 0.83
