Explain the preparation of buffer solution of acetic acid and sodium acetate.
Answers
Buffer Solution Demonstration: Acetic Acid/Acetate vs. Water Compare pH
This demonstration focuses on imparting the concept of a buffer solution to students. Solutions having a pH of 4, 5, 6, 7, 8, 9, 10 are placed in separate labeled Erlenmeyer flasks. Universal indicator is added to each solution. [pH meter is optional]
Distilled water is adjusted to have a pH = 7.0. Half of this solution is placed in beaker "A" . Beaker "A" and its contents are on a stir plate with a magnetic stir bar in the water. Universal indicator solution is added. Next, the demonstrator builds the buffer solution in front of the students starting with deionized water in Beaker "B" and then adding acetic acid. Beaker "B" and its contents are on a stir plate with a magnetic stir bar in the solution. The instructor asks students to estimate the pH of the acetic acid solution and to write the equilibrium equation representing this weak acid system.
CH3COOH(aq) + H2O(l) <=> H3O+(aq)+ CH3COO- (aq) Ka = 1.8 x 10-5
Universal indicator is added to the acetic acid solution and to the students observe the indicator in the acetic acid solution has a color corresponding to acidic.
The instructor asks students what will happen to the pH of the acetic acid solution when sodium acetate is added. Students should invoke LeChatilier's Principle and the common ion effect. The acetate ion is the common ion. When added to the acetic acid system at equilibrium, the acetate will react with some of the hydronium ions, causing the equilibrium to a shift to the left. Since the hydronium ion concentration decreases, the pH should increase (become less acidic).
Next, solid sodium acetate is added to the acetic acid solution until the color of the indicator in the solution is "green" corresponding to pH = 7. Both the water and the acetic acid/acetate solution have the same color and therefore both solutions have same pH.
Small quantities of 010 M HCl and then 0.10 M NaOH are added to water. Students observe the color of the indicator in the water changes dramatically. Small quantities of 010 M HCl and then 0.10 M NaOH are added to the acetic acid/acetate solution. The color of the indicator in the buffer solution slightly changes. Water is not a buffer solution and the acetic acid/acetate solution is a buffer solution.
An acidic buffer is a solution of a weak acid (acetic acid) and its conjugate base pair (sodium acetate) that prevents the pH of a solution from changing drastically through the action of each component with incoming acid or base.
Acetic acid in the buffer solution will react with the addition of sodium hydroxide, NaOH
CH3CO2H(aq) + OH-(aq) –> CH3CO2-(aq) + H2O(l) K=1.8 x 109
The acetate anion in the buffer solution will react with the addition of hydrochloric acid, HCl
CH3CO2-(aq) + H3O+(aq) –> CH3CO2H(aq) + H2O(l) K=5.6 x 104
The steps to prepare buffer solution for acetic acid are:
1) 800 ml of distilled water is to be prepared.
2) To that 800 ml water please add 7.719 g of Sodium Acetate (anhydrous)
3) Then add 0.353 g of Acetic Acid.
4) pH of the solution is to be adjusted by adding HCl or NaOH to the desired amount.
The steps to prepare buffer solution for 0.1 percent of sodium acetate are:
1) 800 ml of distilled water is to be prepared.
2) To that 800 ml water please add 5.772 g of Sodium Acetate (anhydrous)
3) Then add 1.778 g of Acetic Acid.
4) Adjust solution to desired pH using 10 N HCl (exactly pH ≈ 5.0).