Department of EEE
Course Name: Introduction to Chemistry
Course Code: 101
Section No: 01
Student’s Name: Nahian Maisha
Student’s Id: 2016-3-80-016
Experiment No: 04
Name of the Experiment: Determination of the Molar Mass of an unknown
monoprotic acid, HA.
Submitted to: Dr. Thamina Acter
Submission date: August 05,2021
Theory:
A monoprotic acid is an acid that donates only one proton or hydrogen atom per molecule to an aqueous
solution, such as: Hydrochloric acid (HCl), Nitric acid (HNO3), Acetic acid (CH3COOH). The molar mass
of a chemical compound is the mass of a sample of that compound divided by the amount of substance in
that sample, measured in moles. Molar masses are usually expressed in g/mol.
Chemical reactions between acids and bases are used to analyze the quantity of a pure substance in a
mixture. In this experiment, the standardized base, NaOH, is used to determine the molar mass of an
unknown weak acid. Based on the molar mass, you will determine the identity of the unknown weak acid.
Reaction: NaOH(aq) + HA(aq) NaA(aq) + H2O(l)
Since 1 mol of NaOH reacts with 1 mol of HA, the following expression can be written:
(MHA×VHA )= (MNaOH×VNaOH)……………….(1)
Where, MNaOH = Molarity of NaOH
VNaOH = Volume of NaOH = Average burette reading, mL
VHA = Volume of HA
MHA = Molarity of HA
Apparatus:
Burette (50 ml)
CH3COOH
NaOH
Phenolphthalein
Volumetric Flask (100 mL)
Conical Flask
Distilled water
Procedure:
a) Dilution of HA:
Retrieved the 1M CH3COOH solution and a 100mL volumetric flask from the stockroom. In
order to make 0.1M CH3COOH solution, taken 10mL of 1M stock CH3COOH solution,
transfered it to 100mL volumetric flask and diluted it up to the mark with water.
b) Determination of the concentration of HA:
Filled the burette with 0.1M NaOH and recorded the initial burette reading in Table 1.
Taken 10 mL of HA in a conical flask with the help of pipette.
Added 0.1mL of phenolphthalein indicator.
Titrated until the last drop of NaOH solution leaves a permanent pink color in the solution and
recorded the final reading in Table 1
Calculated the difference between two burette readings (initial and final), which is the
amount of NaOH required neutralizing CH3COOH.
LAB WORK:
Table 1: Record of burette readings:
Observation Initial burette Final burette Difference Average
No reading (mL) reading (mL) (mL) (mL)
1 50 39.8 10.2 10.2
Calculation:
Calculate the concentration of HA (CH3COOH) in terms of Molarity by using the following formula from
equation (1):
Concentration of HA, MHA
0.1×10.2
= 10
M
= 0.102 M
Where, MNaOH =0.1M
VNaOH = Volume of NaOH = Average burette reading, mL
VHA = Volume of HA = 10mL
MHA = Molarity of HA =?
So, Concentration of HA, MHA = 0.1 M
or, MHA = 0.1 mol/1000mL
Number of moles of HA, nHA= 0.1mol × (100 mL / 1000 mL) = 0.01 mol
Molar mass of HA, MMHA = 0.60 g /0.01mol = 60g/mol
Percentage Error:
Error =
0.1−0.1
=| 0.1
| × 100%
=0%
Result:
The molarity of monoprotic acid, HA (CH3COOH) is 0.1 M and molar mass of HA is 60 g/mol.
Discussion:
By this experiment learnt many things such as how to determine the molar mass of an unknown acid by
titrating the acid solution against a solution of NaOH of known molarity. Data was collected through
analysis of the titration process and calculations based on titration. The NaOH solution was added from a
burette to a flask until the end point was reached. Experimenters used measured mass of solids and
volume of NaOH solution in the titration process to determine molarity of the NaOH solution. We also
used the known molarity to calculate the moles and molar mass of an unknown acid that was in solution.
If this indicator was not present during the titration process, there would be no visible change as the two
solutions were combined. Both the reactants and the products would appear as clear, colorless liquids.
When a drop of titrant is added, turns dark pink, and then disappears, this shows the reaction in process.
The pink indicates a brief pH change when the titrant is added. When the NaOH solution comes in contact
with the unknown acid solution, the basic product is formed, causing the brief presence of the pink color.
By this lab all value should be taken carefully and learnt many new things.
