8 - Buffers SCH4U – Acid Base Chemistry

Date: ______

Buffers

  • Solutions that resist ______
  • Consists of an ______of a ______and ______
  • Buffers play an important role in biological systems (______)

CH3COO- + H2O ⇋ CH3COOH + OH-

  • If we add HCl, ______is neutralized
  • Equilibrium shifts to ______
  • The addition of base is neutralized by the ______
  • Equilibrium shifts to deal with ______

Calculating pH Buffer Systems

  • Calculate the pH of a buffer system prepared by mixing 20.00 mL of 0.100M CH3COOH with 40.00 mL of 0.100 M CH3COONa. Ka = 1.76x10-5
  • Instead of using an ICE table, we can use the Henderson-Hasselbach Equation

CH3COOH + H2O ⇋ CH3COO- + H3O+


Examples:

  • Calculate pH of a solution made by mixing 100.0 mL of 0.100 M NH3 and 100.0 mL of 0.050 M HCl.
  • Calculate mass of sodium acetate which must be added to 100.0 mL of 0.100 M acetic acid to make a buffer of 4.80. Ka = 1.8 x 10-5
  • Calculate pH of a solution made by mixing 100.0 mL of 0.200 M HNO3 and 50.0 mL of 0.200 M NaOH.


Buffer Capacity

  • A measure of the amount of acid or base which a buffer can ______before it is ______
  • A buffer becomes ______when either all of weak acid is ______by base or all conjugate base is ______by excess acid

CH3COOH + H2O ⇋ CH3COO- + H3O+

  • If enough base is added to ______all the acetic acid, the buffer cannot neutralize any more ______and the addition of base will yield ______
  • Buffer capacity is influenced by the ______of weak acid and conjugate base in solution

Optimum Buffer

  • A buffer which is able to neutralize large quantities of either excess acid or base with ______
  • Prepared by mixing as ______amounts of weak acid and conjugate base

Choosing an Acid for a Buffer

  • The optimum buffer ratio of acid to conjugate base should be as close to one to one as possible
  • Desirable to choose a weak acid with the ______as close to desired _____

Example

  • Which acid would be the best for a pH of 4.3?

Chloroacetic: Ka = 1.35x10-3

Propanoic: Ka = 1.3x10-5

Benzoic: Ka = 6.4x10-5

Hypochlorous: Ka = 3.5x10-8


Titration Curves

  • A plot of pH as a function of volume of titrant added
  • Constructed by adding a ______volume of titrant to an unknown solution
  • Record pH after each addition
  • Common analytical technique to determine an ______
  • Endpoint is ______on a titration curve
  • Equivalence point is the point at which ______is equal to original ______
  • In a good titration, the endpoint is the equivalence point
  • Phenolphthalein is often used

Homework

Page 620

Section 8.5 Questions 1, 3-7

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