Buffered Solutions What is a buffer? Calculations involving Buffers.

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Presentation transcript:

Buffered Solutions What is a buffer? Calculations involving Buffers

What are buffers? Buffers are solutions of a weak conjugate acid- base pair. They are particularly resistant to pH changes, even when strong acid or base is added.

Buffer Solutions Consist of a weak acid and its salt – HF and NaF Consist of a weak base and its salt – NH 3 and NH 4 Cl

How are they resistant to pH changes? How do buffers “work” ? If we mix a weak acid (HA) with its conjugate base (A - ), both the acid and base components remain present in the solution. This is because they do not undergo any reactions that significantly alter their concentrations. The weak acid and weak base remain in the solution with high concentrations since they rarely react with the water.

How do buffers “work”? The acid/conjugate base may react with one another, HA + A - → A - + HA, but when they do so, they simply trade places and the concentrations [HA] and [A - ] do not change. In addition, HA and A - rarely react with water. Weak acid rarely dissociates in water (will rarely lose its proton H + to water). Conjugate base is weak, it rarely steals a proton H + from water.

Example A buffered solution contains 0.50 M acetic acid (K a = 1.8 x ) and 0.50 M sodium acetate. Calculate the pH of this solution.

Equilibrium and Buffers pH of Buffers determined by two factors: #1. value of K a for the weak acid/base (K a ) #2. ratio of concentrations of conjugate acid/ base pair [HA] / [A-] Henderson –Hasselbach Equation

Sample Exercise 17.3 p. 709 Common Ion Effect, neglect x LIST THINK ICE SOLVE

Henderson-Hasselbach Eqn Can be used when the ratio is known [HA]/[A-] Can be used when K a or K b is known (pK a )

Sample Exercise 17.4 p. 710 Work Backwards using K b and [OH-] Get out Buffers Made Easy Handout! Let’s watch and learn!