Assignment Paper

SUNY OLD WESTBURY
CHEMISTRY/PHYSICS DEPARTMENT
PRINCIPLES OF CHEMISTRY 2 CP 2131

EXPERIMENT #8: BUFFERS (DRY PROJECT LAB)

Objectives
(1) To prepare buffer solution at desired pH
(2) To determine the buffer capacity

Introduction
A buffer is a solution that can resist changes to the pH upon the addition of small quantities of an acid or a base. Buffer solutions are pH stabilizers. The ability of buffers to resist pH changes results from the simultaneous presence of a weak (HA) and its conjugate base (A-), or a weak base (B) and its conjugate acid (HB), and this can be explained by the Le Chatelier’s principle.
Suppose a small amount of an acid (H+) is added to a chemical equilibrium (equation 1), the equilibrium will shift to the left to lower the “extra” amount of H+. Similarly, when a small amount of a base (OH-) is added, the equilibrium will shift to the right to release more H+.

Click here to watch the mechanism of a buffer solution
https://www.khanacademy.org/test-prep/mcat/chemical-processes/acid-base-equilibria/v/buffer-solutions

The Henderson-Hasselbalch equation, equation 3, can be used to calculate the pH of a buffer solution.

Buffer capacity (β) is a measure of the buffer’s ability to maintain pH, and is calculated using equation 4.

Where Δn is the number of moles of strong base or strong acid added per liter, and ΔpH is the change in pH observed. Thus, β has units of mol/L.pH.

Click here to watch a demonstration of buffer capacity

Buffer capacity can be determined by titrating the buffer with a strong acid or a strong base. For example, suppose you have 20.00 mL of pH 7.40 buffer and observed an increase in pH from 7.40 to 8.00 when 15.00 mL of 0.010M NaOH was added. Then the buffer capacity can be calculated as:

The best buffer capacity is achieved when [A-] = [HA], then pH = pKa ± 1.The higher the concentration of [A-] + [HA], the larger the buffer solutions, it is important to select a weak acid with desired pKa, and use concentrations that satisfy buffer capacity. The pKa values and possible acid-base pair you may use for this experiment are summarized in Table 1

Safety
In an actual experiment, the solutions should be handled with care and disposed of using the appropriate waster containers.

Procedure
Because this is a project lab, you will design your own experimental procedure. Specifically, using chemicals provided (see Supplies), design procedure for preparing 25.0 mL buffer solution at a pH of 7.0 and 10.0 with buffer concentration of 0.010 M. Also, describe how the buffer capacity can be determined. Show your calculations.

Example Lab Design:
Using some of the chemical listed above, design a procedure that enables you to prepare a 50.0 mL buffer solution at a pH of 5.0 and concentration of 0.010 M. Show your calculations.

Calculations:

Click to watch a complete experiment and calculation in a buffer experiment

Technique you may want to learn or review
o The use of pH meter
o Titration
o Use of volumetric glassware
o Dilution

Assume the following supplies are available
Acetic acid, CH3COOH (5 mL 6.0M, MW = 60.06 g/mol)
Sodium acetate (CH3COONa) (0.1g, MW = 82.03 g/mol)
Sodium bicarbonate (NaHCO3) (0.1g, MW = 84.01 g/mol)
Sodium carbonate (Na2CO3) (0.1g, MW = 105.99 g/mol)
Sodium phosphate (Na3PO4) (0.1g, MW = 163.94 g/mol)
Sodium phosphate dibasic (Na2HPO4) (MW = 141.96 g/mol)
Sodium phosphate monobasic (NaH2PO4) (MW = 119.98 g/mol)
NaOH: 0.100 M and 0.0100 M for titration
HCl: 0.100 M and 0.0100 M for titration
pH meter, electrode and standard buffer solution (pH 4, 7, and 10)
50-ml burette
10-mL transfer pipette
100-mL volumetric flask

Use the information above to prepare a complete lab report that will include the following sections: Objectives, Introduction, Experimental Procedure, Results, Discussions, Conclusions, and Possible Safety Considerations.