E 02 E 12 E 02 695E 03 7 E 07 100E

E 02 e 12 e 02 695e 03 7 e 07 100e

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1.00E-09 1.29E-02 8.14E-05 5.13E-12 2.00E-02 1.31E-02 6.95E-03 7 2.00E-02 1.00E-07 1.00E-07 7.97E-03 5.03E-03 3.17E-08 2.00E-02 1.80E-02 1.97E-03 9 2.00E-02 1.00E-09 1.00E-05 2.03E-04 1.28E-02 8.07E-06 2.00E-02 2.58E-02 -5.82E-03 11 2.00E-02 1.00E-11 1.00E-03 1.94E-06 1.22E-02 7.71E-04 2.00E-02 2.78E-02 -7.77E-03 13 2.00E-02 1.00E-13 1.00E-01 2.82E-09 1.78E-03 1.12E-02 2.00E-02 1.37E-01 -1.17E-01 By varying the pH of the solution, and therefore [H + ], we can determine changes in the concentrations of all the other species. Since aqueous solutions are electrically neutral, the adequate pH would be the pH corresponding to a net charge of zero (or the closest to zero) Refine the pH range (see next slide) 1 4 2 4 3 4 Na H * PO 2 * PO 3 * PO H w K TOT TOT TOT TOT a a a
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pH Na H OH H2PO4 HPO4 PO4 Sum (+) Sum (-) Net Charge 7 2.00E-02 1.00E-07 1.00E-07 7.97E-03 5.03E-03 3.17E-08 2.00E-02 1.80E-02 1.97E-03 7.1 2.00E-02 7.94E-08 1.26E-07 7.24E-03 5.75E-03 4.57E-08 2.00E-02 1.88E-02 1.25E-03 7.2 2.00E-02 6.31E-08 1.58E-07 6.50E-03 6.50E-03 6.50E-08 2.00E-02 1.95E-02 5.00E-04 7.3 2.00E-02 5.01E-08 2.00E-07 5.75E-03 7.24E-03 9.12E-08 2.00E-02 2.02E-02 -2.45E-04 7.21 2.00E-02 6.17E-08 1.62E-07 6.43E-03 6.57E-03 6.73E-08 2.00E-02 1.96E-02 4.25E-04 7.22 2.00E-02 6.03E-08 1.66E-07 6.35E-03 6.65E-03 6.96E-08 2.00E-02 1.96E-02 3.50E-04 7.23 2.00E-02 5.89E-08 1.70E-07 6.28E-03 6.72E-03 7.21E-08 2.00E-02 1.97E-02 2.75E-04 7.24 2.00E-02 5.75E-08 1.74E-07 6.20E-03 6.80E-03 7.46E-08 2.00E-02 1.98E-02 2.01E-04 7.25 2.00E-02 5.62E-08 1.78E-07 6.13E-03 6.87E-03 7.71E-08 2.00E-02 1.99E-02 1.26E-04 7.26 2.00E-02 5.50E-08 1.82E-07 6.05E-03 6.95E-03 7.98E-08 2.00E-02 1.99E-02 5.15E-05 7.27 2.00E-02 5.37E-08 1.86E-07 5.98E-03 7.02E-03 8.25E-08 2.00E-02 2.00E-02 -2.29E-05 1 4 2 4 3 4 Na H * PO 2 * PO 3 * PO H w K TOT TOT TOT TOT a a a
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SUMMARY 1. Acids and bases are, respectively, substances that increase and decrease the activity of H + in solution. 2. The strength of an acid is quantified by its dissociation constant K a 3. The strength of a base by its basicity constant K b , and the product K a K b is the dissociation constant for water. 4. The fractional distribution of a compound among its acid and base forms depends on solution pH and is independent of the total amount of the compound in the system. 5. The determination of the pH in a equilibrium solution that has been prepared with known inputs, or the determination of solution speciation in a solution of known pH involves writing and solving a fairly straightforward set of simultaneous equations.
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log C - pH Diagrams for H 3 PO 4 = 1.0
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PRACTICE
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Example Practice Problem-1 : Using the spreadsheet approach and the mass balance and charge balance equations, determine the pH of a solution obtained when a weak acid, acetic acid ( H 3 C-COOH or HAc ), is added to water to a final concentration of 10 -2 M – Assume ideal solution Show the following: 1. Species present 2. Relevant chemical reactions 3. Equilibrium relationships 4. Mass balance (MB) equation 5. Charge balance (CB) equation 6. Solve for pH using the spreadsheet approach A Simple Spreadsheet Analysis for Determining Acid/Base Speciation
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Weak Acid -Base Systems Example :
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  • Fall '08
  • CHADIK
  • pH, kA, propionic acid

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