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Prelaboratory Work
Before the experiment in the laboratory, you should be able to answer these questions.
- What is the definition of standardization?
- What is the definition of titration?
- What is the definition of molarity?
- Write the difference between equivalence points and end points.
- Find the molarity of a solution that contains 3.78 g of H 2 C 2 O 4 .2H 2 O in 100 mL of solution?
CEAC 103
GENERAL CHEMISTRY
Experiment 5
Titration of Acids and Bases
Purpose: To become familiar with the techniques of titration, a volumetric
method of analysis; to determine the molarity and pH of an acid solution.
Theory
According to Arrhenius, acid is a chemical substance which gives hydronium ion, H 3 O+, when dissolved in water. Also, Bronsted-Lowry defined that acid acts as a proton donor. On the other hand, Arrhenius defined base in a way that it gives hydroxide ion, OH-, when dissolved in water. According to Bronsted-Lowry, base acts as a proton acceptor.
One of the most common and familiar reactions in chemistry is the reaction of an acid with a base. This reaction is named as neutralization reaction , and the essential feature of this process in aqueous solution is the combination of hydronium ions with hydroxide ions to form water.
H 3 O+(aq) + OH-(aq) 2H 2 O(l)
In this experiment you will perform this reaction to determine accurately the concentration of a sodium hydroxide solution that you have prepared. The process of determining the exact concentration (molarity) of a solution is called standardization. Next you will measure the concentration of the unknown acid solution. For this purpose you are expected to measure the volume of your standard base that is required to exactly neutralize the unknown acid solution. The technique of accurately measuring the volume of a solution required to react with another reagent is termed titration.
During titration, you will use an indicator solution to understand whether you could neutralize your acid with a base or vice versa. Indicators change colors at different pH values. For example, phenolphthalein changes color from colorless to pink at a pH of about 9; in slightly more acidic solutions it is colorless, whereas, in more alkaline solutions it is pink. The color change is termed the end point of the titration.
The point at which stoichiometrically equivalent quantities are brought together is known as the equivalence point of the titration. It should be noted that the equivalence point in a titration is a theoretical point.
Molarity (M) is used to define concentration of a solution more clearly ,and it is defined as the number of moles of solute per liter of solution, or the number of millimoles of solute per milliliter of solution:
M=
mL
mmol liter
mole volumeofsolution
moles solute (^)
3
3
10
[1]
Strong Acid-Base Titration
As an example for neutralization reaction between strong acid (e.g. HCl) and a strong base (e.g. NaOH);
HCl (aq) + NaOH (aq) → NaCl (aq) + H 2 O (l)
As a result, for a monoprotonic acid and base at the end point;
MacidVacid = MbaseVbase
In this experiment, we use an acid-base indicator, phenolphthalein to determine the end point in
the titration. We choose an indicator such that its color change occurs as closely as possible to the equivalence point.
Materials
Sodium hydroxide (NaOH) 600 mL beaker Ring stand Hydrochloric acid (HCl) 250 mL Erlenmeyer flasks Buret clamp Phenolphthalein solution 50 mL buret Balance Wash bottle 500 mL erlenmeyer flask
Procedure
PART A: Standardization of Sodium Hydroxide Solution
1. Fill the buret with the NaOH solution and remove the air from the tip by running out some of the liquid into an empty beaker. Make sure that the lower part of the meniscus is at the zero mark or slightly lower. 2. Take 15.00 mL of standard HCl solution into a clean Erlenmeyer flask and add a few drops of phenolphthalein solution. 3. Start to add the sodium hydroxide solution slowly to your flasks of HCl solution while gently swirling the contents of the flask. As the sodium hydroxide solution is added, a pink color appears where the drops of the base come in contact with the solution. This coloration disappears with swirling. As the end point is approached, the color disappears more slowly, at which time the sodium hydroxide should be added drop by drop. The end point is reached when the colour of the solution turns from colorless to pink. 4. Read the buret and record it. Repeat this procedure for two times. From the data you obtain in the three titrations, calculate the molarity of the sodium hydroxide solution to four significant figures.
DATA SHEET
Titration of Acids and Bases
Student’s Name : Date:
Laboratory Section/Group No :
Assistant’s Name and Signature :
A. Standardization of Sodium Hydroxide Solution
- Molarity of HCI solution : ……………
- Volume of HCl solution: ……………
- Volume of NaOH solution used for the titrations 1st: …….2nd: ……..3rd: ……..
- Molarity of NaOH solution: …………….
B. Analysis of an Unknown Acid I
- Volume of unknown acid solution: ……………
- Volume of NaOH solution used for the titration : …………….
- Molarity of unknown acid I:.....................
C. Analysis of an Unknown Acid II
- Volume of unknown acid solution: ……………
- Volume of NaOH solution used for the titration : …………….
- Molarity of unknown acid II:.....................
