Unit 13a

Investigate acid-base equilibria in order to
understand buffer action and to optimise acid-base titration
procedures



ACID AND BASE STRENGTH CALCULATION TEST

To calculate pH, H+ ion concentration, and acid dissociation constants (Ka) for
different acids, bases, and buffer solutions, do simple calculations. Fill out the
"Acid-Base Calculations Test" completely, using the correct units and accuracy for
each answer. Make that buffer solutions, weak acids, strong alkalis, and strong acids
are all included in the computations. Practice measuring the pH of various solutions
and deriving the [H+] value from pH. Give them plenty of practice in advance of a
quick test, along with a formula sheet. Ten questions covering two computations of
each kind (strong acids, strong alkalis, weak acids, and buffer solutions) should be
included in the practice exam.


PROCEDURE OF ACETIC ACID-SODIUM ACETATE BUFFER

Reagents Needed:


● Acetic Acid 0.2M: To manufacture up to 100 ml, combine 1.5 ml of glacial
acetic acid with distilled water.
● Solution of Sodium Acetate: In 100 millilitres of distilled water, dissolve 0.64
grams of sodium acetate or 2.72 grams of sodium acetate trihydrate.

Procedure:
1. Empty a 100 ml standard flask with precisely 36.2 ml of the sodium acetate
solution using a pipette.
2. Fill the flask with 14.8 ml of glacial acetic acid.
3. Adjust the volume with distilled water to 100 millilitres.
4. A 0.2 M acetic acid and sodium acetate buffer solution is the end product of
this procedure.
5. Using a pH metre, find the solution's pH.




PH Measurement:

, ● Standardise the pH metre with pH buffer.
● Wash the electrode with distilled water and introduce it into the 0.2 M acetic
acid-sodium acetate buffer solution.
● Record the pH of the solution, which is observed to be 4 initially, and adjusted
to 4.6 using 5N NaOH.

Result:
● A buffer solution was prepared by mixing 36.2 ml of sodium acetate solution
with 14.8 ml of glacial acetic acid.
● The initial pH reading was 4, which was adjusted to 4.6 using 5N NaOH.

STRONG ACID CALCULATION

Calculate pH of 0.1 M HCl solution
1. solution= pH=−log[H+]=−log(0.1)=1.0
2. Result=PH = 1.0

Calculate concentration of H+ ions in 0.01 M H₂SO₄
solution

1. Solution= [H+]=0.01
2. Result= [H+]=0.01 M

STRONG ALKALI

Determine pH of 0.05 M NaOH solution

1. Solution= Since NaOH is a strong base,
Ph=14−log⁡[OH−]=14−log⁡(0.05)=12.30
2. pH=14−log[OH−]=14−log(0.05)=12.30
3. Result= pH = 12.30

Find concentration of OH- ions in 0.025 M KOH solution

1. Solution: [OH−]=0.025
2. Result: [OH−]=0.025 M

,WEAK ACIDS

Calculate pH of 0.1 M acetic acid (Ka =1.8×10−5 1.8×10−5) solution

● Solution= Let x be the concentration of+H+ions dissociated from acetic
acid.Ka=X2/0.1 X3 , solving for x, we get x square root x we get x times
Ka=0.1×1.8×10−5≈1.34×10−3 x=0.1×Ka=0.1×1.8×10−5​≈1.34×10−3. Thus,
Ph=−log⁡(1.34×10−3)=2.87pH=−log(1.34×10−3)=2.87
● Result= pH 2.87

Determine concentration of H+ ions in 0.05 M HF (Ka =7.2×10−47.2×10−4) solution

● Solution= Let x be the concentration of H+ions dissociated from HF
Ka=X2/0.05-X3,
solving for X we getX= square root
0.05×Ka=0.05×7.2×10−4≈8.48×10−3x=0.05×Ka​=0.05×7.2×10−4​≈8.48×10−3.
Thus,[H+]=8.48×10−3[H+]=8.48×10−3M

● Result= [H+]=8.48×10−3M

Buffer Solutions



Buffer Solution Initial pH Measurement pH Adjustment

Volume of Sodium 36.2 ml N/A
Acetate Solution

Volume of Glacial Acetic 14.8 ml N/A
Acid

Initial pH Reading 4 N/A

pH Adjustment with 5N N/A 4.6
NaOH


Aim:
The objective is to make an acidic buffer solution, figure out its pH, and use a pH
metre to compare the calculated and observed pH values. Furthermore, to track the
pH changes that occur when little amounts of strong acid or strong alkali are added
to the buffer solution.

, Principle:
The electrical potential that a pair of electrode pins in a solution develops is
measured by a pH metre. An electrode device sensitive to variations in the solution's
H+ ion concentration is used to detect pH. The electrode system is made up of a
series of electrodes whose potential increases in proportion to the pH (solution's H+
content).

Comparison of calculated and measured pH

The pH of the buffer solution is found to be roughly 4.74 by calculation, which agrees
well with the pH of 4.6 that was observed. This shows how accurate the pH metre is
and how well the buffer solution keeps the pH stable.

Effects of Adding Strong Acid or Strong Alkali

When modest amounts of strong acid or strong alkali are added to the buffer
solution, the pH of the mixture does not vary much when compared to when the
same amounts are added to pure water. This highlights the buffer solution's function
in preserving pH stability in a variety of chemical settings by demonstrating its
resistance to significant pH shifts.

Buffer Solution Preparation:
● Weak Acid: Glacial acetic acid (CH3COOH)
● Conjugate Base: Sodium acetate (CH3COONa)

Initial Concentrations:
● Volume of sodium acetate (CH3COONa) = 36.2 ml
● Volume of glacial acetic acid (CH3COOH) = 14.8 ml

Henderson-Hasselbalch Equation:
● pH=pKa+log⁡([A−][HA])
● pH=pKa+log(A-/HA)
● Since glacial acetic acid and sodium acetate are a conjugate acid-base
pair, the pKa of acetic acid can be used.

Calculate Theoretical pH:
● Acetic acid pKa = 4.76 (approximate value)
● Calculate concentrations of CH3COOH and CH3COO^-:
● Given: Volume = 36.2 ml (0.0362 L) for CH3COONa and 14.8 ml
(0.0148 L) for CH3COOH