Chemistry 104 Laboratory

 

 

This Week

Answer Keys

Gen Chem Lab

Periodic Table

 

 

 

á       Expt 5:  pH Titration

 

Next Week

á       Expt 6:  Solubility and Thermodynamics

á       Quiz 4:  Expt 1

 

Notes for Today
  • Overall Goals
    • Explore acid-base equilibria through titration curves
    • Use titration curve to identify an unknown acid or base

 

 

  • pH Titration Curve
    • Plot of how pH changes over the course of an acid-base rxn

          e.g., HCl + NaOH --> H2O + NaCl

 

  

 

 

    • Molar mass comes from equivalence point information
    • pK comes from pH at half equivalence

 

  • pH at any point can be predicted

Ka = [H+][A-]/[HA]

-log(Ka) = -log[H+] + {log([A-]/[HA])}

pKa = pH - log([A-]/[HA])

pH = pKa + log([A-]/[HA]

 

Four Important Regions/Points In Any HA-NaOH Titration

 

 

  • Point 1:  Start of the Titration
    • Only HA; no NaOH added yet.
    • pH determined by "strength" of HA.
    • How does Ka influence pH?
  • Region 2:  Between the Start and the Equivalence Point
    • HA and NaOH are reacting.
    • More A- forms.
    • pH slowly increases.  Why slowly?
    • ÔHalf equivalence pointÕ is exactly midway between start and equivalence point.
  • Point 3:  The Equivalence Point
    • Enough NaOH has been added to react with all HA

moles NaOH added = moles HA initially present

No NaOH, HA remain (rxn complete)

Only NaA + water remain

    • pH is determined by the strength of A-.
    • How does Ka of HA influence pH here?
  • Region 4:  After the Equivalence Point
    • More NaOH is added, but there is no HA for rxn.
    • The NaOH simply raises the pH.

 

  • Problem:  For the two titration curves below, account for any differences in
    • Initial pH
    • Equivalence point (pH, volume NaOH)
    • Half equivalence point location (pH, volume NaOH)

0.0050 mole HA:  HA + NaOH --> H2O + NaA

0.0050 mole HCl:  HCl + NaOH --> H2O + NaCl