Equilibria of Weak Acids

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Equilibria of Weak Acids BasesPart III
Quantitative Analysis of Salts

• Dr. C. Yau
• Fall 2009

Based on Brady Senese 5/eChap. 16 Sec 3
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Example 16.10 p.659
What is the pH of a 0.10 M solution of NaOCl? For
HOCl, Ka 3.0x10-8
Example 16.11 p.660
What is the pH of a 0.20 M solution of
hydrazinium chloride N2H5Cl? Hydrazine N2H4 is a
weak base with Kb 9.6x10-7
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Example 16.12 p. 662
Will an aqueous solution that is 0.20 M NH4F be
acidic, basic or neutral? Do Pract Exer 19,
20, 21, 22, 23 p.661
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Summary of what we did in examples 16.10, 16.11
16.12 Example 16.10 Calculation for pH of salt
made from strong base and weak acid
(NaOCl). Example 16.11 Calculation for pH of
salt made from weak base and strong acid
(N2H5Cl). Example 16.12 Analysis of salt made
from weak base and weak acid (NH4F).
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Section 16.4Simplifications fail for some
equilibrium calculations
What are the authors talking about?? They are
talking about when you assume x to be
negligible. So what do you do when x is not
negligible? Two solutions ???
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A quick way to decide whether to apply the
assumption HAinitial gt 400 x Ka Example
16.11 0.20 M N2H5 Ka 1.0x10-8 400 x Ka
4.0 x 10-6 0.20 gt 400 Ka Assumption is justified.
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Example 16.10 0.10M NaOCl OCl- is acting as a
base. We look at Kb of OCl- (which must be
calculated from Ka of HOCl)
Kb is 3.3x10-7
400 x Kb 1.3x10-4 0.10 M gt 400 Kb so assumption
is justified.
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Example 16.13 p. 663
Chloroacetic acid, HC2H2O2Cl, is used as an
herbicide and in the manufacture of dyes and
other organic chemicals. It is a weak acid with
Ka 1.4x10-3. What is the pH of a 0.010 M
solution of HC2H2O2Cl?