Chapter 5 LAB

• Discussion Outline


  • Titration Overview
  • Titration Calculations
  • Monoprotic Titration
  • Strong Acid – Strong Base
  • Weak Acid – Strong Base
  • Weak Base – Strong Acid
  • Buffers
  • Acid-Base Titration Indicators
  • Titration of Acid-Base Mixtures

  
• Titration


  Volumetric analysis in which substances react to completion, often observed using chemical indicators
• Neutralization titrations


  Widely used to determine the amounts of acids and bases, and to monitor the progress of reactions that produce or consume hydrogen ions
• Analyte


  The substance without known concentration or identity
• Titrant / Standard


  The substance with accurately known concentration

  
• Indicators


  Substances that produce distinct color at certain pH
• Equivalence point


  The point at which the reaction is complete; usually computed using stoichiometry
• End point


  An estimate position at which an equivalence point condition is observed; usually done in experiment
• Titration calculations would be easier to approach if you know the species present in your solution. This way, you will know which of the many equations is needed.
• If there is a presence of acid-conjugate base or base-conjugate acid pairs, use the Henderson-Hasselbalch equation.
• Upon the addition of acid, the moles of base decreases. Similarly, upon the addition of base, the moles of acid decreases.
• Addition of two liquids (titrant and analyte) will change the concentrations of the species in aqueous media. Always use the dilution equation.
• Knowledge of equilibrium calculations would be helpful in some problems.
• Monoprotic titrations


  Titrations whose ratio of analyte and titrant is 1:1

  
• Monoprotic titrations


  • Strong acid – strong base titration
  • Weak acid – strong base titration
  • Weak base – strong acid titration
• Monoprotic SA – SB titration


  1. Initial stage

  2. Pre-Equivalence Point stage

  3. Equivalence Point stage

  4. Post-Equivalence Point stage
• Monoprotic SA – SB titration


  Ruled by basic stoichiometry and dilution calculations

  
• Weak acid – strong base titration

  Weak base – strong acid titration


  Ruled by equilibrium equations
• 10 mL of 0.1 M HCl is titrated with 0.05 M NaOH
• Monoprotic SB – SA titration


  1. Initial stage

  2. Pre-Equivalence Point stage

  3. Equivalence Point stage

  4. Post-Equivalence Point stage
• Monoprotic SB – SA titration


  Ruled by equilibrium equations
• 50.00 mL of 0.0500 M NaOH is titrated with 0.1000 M HCl
• 25.00 mL of 0.0250 M NaOH is titrated with 0.050 M HCl
• Monoprotic WA – SB titration

  1. Initial stage

  2. Pre-Equivalence Point stage

  3. Equivalence Point stage

  4. Post-Equivalence Point stage
• 50.00 mL of 0.1000 M acetic acid (HOAc, Ka = 1.8×10-5) is titrated with 0.1000 M sodium hydroxide
• 20 mL of 0.2 M formic acid (HCOOH, Ka = 1.8×10-4) is titrated with 0.1 M NaOH
• Monoprotic WB – SA titration

  1. Initial stage

  2. Pre-Equivalence Point stage

  3. Equivalence Point stage

  4. Post-Equivalence Point stage
• 50.00-mL aliquot of 0.0500 M NaCN (kb = 1.61×10-5) is titrated with 0.1000 M HCl
• 30 mL of 0.10 M methylamine (CH3NH2 Kb = 4.8×10-4) is titrated with 0.1 M HCl
• Buffers


  Solutions that resist changes in pH upon the addition of small amounts of an acid or base
• Sodium hypochlorite (NaOCl, the active ingredient of almost all bleaches) was dissolved in a solution buffered to pH 6.20. Find the ratio [OCl-]/[HOCl] in this solution.
• Find the pH of a 1.00-L aqueous solution prepared with 12.43 g of tris (FM 121.135) plus 4.67 g of tris hydrochloride (FM 157.596).
• If we add 12.0 mL of 1.00 M HCl to the solution in the previous example, what will be the new pH?
• How many milliliters of 0.500 M NaOH should be added to 10.0 g of tris hydrochloride to give a pH of 7.60 in a final volume of 250 mL?
• Acid-Base Titration Indicators


  Organic acids and bases which change color depending on the pH of the solution
• Acid-Base Titration Indicators


  • Alizarin Yellow
  • Phenolphthalein
  • Bromothymol Blue
  • Bromocresol Green (BCG)
  • Methyl Orange
• Titration curves for HCl with NaOH. Curve A: 50.00 mL of 0.0500 M HCl with 0.1000 M NaOH. Curve B: 50.00 mL of 0.000500 M HCl with 0.001000 M NaOH.
• Curve for the titration of acetic acid with sodium hydroxide. Curve A: 0.1000 M acid with 0.1000 M base. Curve B: 0.001000 M acid with 0.001000 M base.
• The effect of acid strength (dissociation constant, Ka ) on titration curves. Each curve represents the titration of 50.00 mL of 0.1000 M acid with 0.1000 M base.
• The effect of base strength (Kb) on titration curves. Each curve represents the titration of 50.00 mL of 0.1000 M base with 0.1000 M HCl.