0629AC91-1139-4F80-BC75-F688436200DE

Lab 4: Buffer Solution

High Pre-Lab

• What is the pH of 0.5 M HCl solution?

  • pH ≈ 0.30

  • Calculation: pH = -log[0.5] = 0.30

• What is the pH of 0.75 M NaOH solution?

  • pH ≈ 13.1

  • Calculation:

    • pOH = -log(0.75) = 0.12

    • pH = 14 - 0.12 = 13.1

• Hydrogen ion concentration of HCl solution with a pH of 3.0?

  • Hydrogen ion concentration: 0.001 M

  • Calculation:

    • pH = -log[H+]

    • [H+] = 10^(-3) = 0.001 M

• pH of 0.50 M HC2H3O2 and 0.25 M NaC2H3O2?

  • Given: Ka of HC2H3O2 = 1.8 x 10^-5

  • Calculation:

    • pKa = -log(1.8 x 10^-5) = 4.74

    • pH = pKa + log{[NaC2H3O2]/[HC2H3O2]}

    • pH = 4.74 + log(0.25/0.5) = 4.74 - 0.301 = 4.439 ~ 4.44

• pH of 0.25 M NH4Cl and 2.5 M NH3?

  • Given: Kb of NH3 = 1.8 x 10^-5; Pkb = -log(1.8 x 10^5) = 4.74

  • Calculation:

    • pH = pKb + log{[NH4Cl]/[NH3]}

    • pH = 4.74 + log(0.25/2.5) = 4.74 + 0.1 = 4.84

Post Lab Buffer Solution

• Abstract:

  • Objective: Prepare various buffer solutions and determine their buffer capacity.

  • Experiment involved adding HCl and NaOH to a buffer solution at pH 5.

  • Noted changes in pH:

    • HCl lowered pH

    • NaOH raised pH

  • Importance: Knowing which chemicals maintain pH in buffer solutions is crucial for efficiency.

• Introduction:

  • Field: Analytical Chemistry

    • Focuses on extraction, identification, quantification of unknown samples.

    • Critical for science and industry, ensuring precision and accuracy.

    • Practical skills developed include handling and preparing chemical solutions.

  • Buffer Solutions:

    • Resist pH changes with additions of acids/bases.

    • Experiments assess buffer capacity and effectiveness.

Methods

• Materials:

  • Acetic Acid (0.10M, 0.5M)

  • Sodium Acetate

  • Standard buffer solutions (pH 4 & 7)

  • 100 ml volumetric flask

  • pH meter, buret clamp, burets, stir bar, 250 ml beakers, universal indicator

• Procedure:

  • First solution: 1.52g sodium acetate + 100ml 0.1M acetic acid

  • Resulting pH of sodium acetate and acetic acid: 5.04

  • Added HCl: pH = 4.99 (24.5°C)

  • Added NaOH: pH = 5.11 (25.7°C)

  • Comparison using tap water:

    • pH of tap water: 7.91 (30.4°C)

    • Added NaOH: pH = 12.84 (27.8°C)

    • Added HCl: pH = 2.88 (31.4°C)

Results

• Addition of HCl and NaOH minimally affected pH of buffer solution, confirming buffer capacity.

Page 4: Calculations

• pH of buffer solution (0.1M Acetic Acid, 0.05M Sodium Acetate):

  • pH = pKa + log{[NaC2H3O2]/[HC2H3O2]}

  • pH = 4.74 + log(0.1/0.05)

  • pH = 4.74 + 0.301 = 5.04

• Moles Calculations:

  • Moles of Acetic Acid: 100ml x 0.100 moles/1000ml = 0.0100 moles

  • Moles of Sodium Acetate: 152ml x 0.05moles/1000ml = 0.0076 moles

  • Moles of HCl: 1.00ml x 1.00moles/1000ml = 0.001moles

  • Moles of NaOH: 1.00ml x 1.00moles/1000ml = 0.001moles

  • pH after adding HCl/NaOH to buffer:

    • pH = 5.04 + log(0.0076 - 0.001)/(0.01 + 0.001) = 5.04 + log(0.0066)/(0.011)

    • Final pH = 4.81

• Buffer Capacity:

  • pH = 4.74 + log(1/10) → pH = 3.74

  • pH = 4.74 + log(10/1) → pH = 5.74

Conclusion

• Experiment successfully created a buffer solution with sodium acetate and acetic acid.

• The added chemicals (NaOH and HCl) did not raise or lower the pH beyond the buffer's capacity range.