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pH of (a)Weak Acid Solutions, (b) Bases, (c) Polyprotic Acids, and (d) Salts

Lecture 21. pH of (a)Weak Acid Solutions, (b) Bases, (c) Polyprotic Acids, and (d) Salts. Chemistry 142 B Autumn 2004 J. B. Callis, Instructor. Calculation of the pH of Weak Acid Solutions - A Systematic Approach (1).

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pH of (a)Weak Acid Solutions, (b) Bases, (c) Polyprotic Acids, and (d) Salts

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  1. Lecture 21 pH of (a)Weak Acid Solutions, (b) Bases, (c) Polyprotic Acids, and (d) Salts Chemistry 142 B Autumn 2004 J. B. Callis, Instructor

  2. Calculation of the pH of Weak Acid Solutions - A Systematic Approach (1) • Problem 21-1: What is the pH of a solution of 1.00 M nitrous acid, Ka = 4.0 x 10-4 • Major species in solution: HNO2 and H2O • Which species can generate H+ ions? • HNO2(aq) = H+(aq) + NO2-(aq) Ka = 4.0 x 10-4 • H2O(aq) = H+(aq) + OH-(aq) Kw = 1.0 x 10-14 • Ignore contribution from water, Ka >> Kw

  3. Calculation of the pH of Weak Acid Solutions - A Systematic Approach (2) • The equilibrium expression is The initial concentration are

  4. Calculation of the pH of Weak Acid Solutions - A Systematic Approach (3) Let x be the change in concentration of HNO2 that is required to achieve equilibrium. Then the equilibrium concentrations are:

  5. Calculation of the pH of Weak Acid Solutions - A Systematic Approach (4) We rearrange this equation to yield a second order polynomial:

  6. Calculation of the pH of Weak Acid Solutions - A Systematic Approach (5) For this example, the solutions are: x = Only the first solution is valid because it leads to all positive concentrations. [H+] = [NO2-] = [HNO2] =

  7. Calculation of the pH of Weak Acid Solutions - A Systematic Approach (6) Question 1(a): What is the pH of this solution? ans: Question 1(b): Were we correct to neglect H+ from the water ans: Question 1(c): What % of the acid is ionized? ans:

  8. Problem 21-2: Calculate of the pH of a Mixture of Weak Acids • Calculate the pH of a mixture of 1.00 M of phenol (Ka = 1.6 x 10-10 and 5.00 M acetic acid (Ka = 1.8 x 10-5). • Major Species in Solution: phenol (HPhe), acetic acid (HAc) and H2O • Which Species Can Generate H+ ions? • HAc(aq) = H+(aq) + Ac-(aq) KHAc = 1.8 x 10-5 • HPhe(aq) = H+(aq) + Phe-(aq) KHPhc = 1.8 x 10-10 • H2O(aq) = H+(aq) + OH-(aq) Kw = 1.0 x 10-14 • Ignore contribution from water and phenol, • KAc >> KHPhe >> Kw

  9. Problem 21-2: Calculate of the pH of a Mixture of Weak Acids (2) Focusing on the Acetic Acid equilibrium: x = pH =

  10. Problem 21-2: Calculate of the pH of a Mixture of Weak Acids (3) How much Phe- is generated?

  11. Problem 21-3: Find the Ka of a weak acid from % dissociation. If 0.10 M propanoic acid dissociates 1.1%, what is Ka?

  12. Bases Definition (Bronsted-Lowry) – a proton acceptor. Strong bases dissociate completely. (e.g. metal hydroxides from Groups 1A and 1B. NaOH(s) -> Na+(aq) + OH-(aq)

  13. Problem 21-4: Calculate the pH of a solution of 3.0 x 10-3 M Ca(OH)2(aq)

  14. Note: A base doesn’t have to contain OH-, it just needs to be able to accept H+, e.g. aqueous ammonium. NH3(aq) + H2O(l) = NH4+(aq) + OH-(aq) Many nitrogen containing compounds are bases. General reaction: B(aq) + H2O(l) = BH+(aq) + OH-(aq) base acid conj. Acid conj. base

  15. Problem 21-5: What is the pH of 1.5 M Dimethylamine (CH3)2NH (Kb = 5.9 x 10-4). Let x be the amount of dimethylamine that has dissociated.

  16. Problem 21-5 (cont.)

  17. Polyprotic Acids Can furnish more than one proton per molecule of acid. They do this in a step-wise manner. Example: oxalic acid: H2C2O4(aq) = H+(aq) + HC2O4-(aq) Ka1 = 5.6 x 10-2 HC2O4-(aq) = H+(aq) + C2O42-(aq) Ka2 = 5.4 x 10-5

  18. Problem 21-6 – Calculate the pH of 0.050 M Ascorbic Acid (Ka1 = 1.0 x 10-5;Ka2 = 5.0 x 10-12). Let x be the amount of [H+] that is produced. Assume that it all comes from the first ionization. Then let the [H+] determine the amount of the doubly ionized base, Asc2-.

  19. Acid-Base Properties of Salts • Salt – an ionic compound that dissolves in H2O to give ions. Sometimes the ions can behave as acids or bases. • Anions that correspond to strong acids, e.g. Cl- and NO3- are weak, weak bases. Also, cations from strong bases, e.g. Na+, K+ are weak, weak acids. • Salts that consist of cations from strong bases and anions from strong acids produce neutral solutions (pH= 7). • Salts of weak acids produce basic solutions. • Salts of weak bases produce acid solutions.

  20. Problem 21-7: What is the pH of 0.25 M Sodium Acetate (NaAc)? Let x be the amount of acetic acid that is formed by the following reaction: Ac-(aq) + H2O(l) = HAc(aq) + OH-(aq).

  21. Problem 21-7 (cont.)

  22. Answers to Problems in Lecture 21 • [H+] = [NO2-] = 0.020 M; [HNO2] = 0.98 M (a) pH = 1.70 (b) Yes (c) 2.0 % • pH = 2.023; Very little dissociation of HPhe ;Very little H+ from HPhe. • pH = 12.48 • 5.0 x 10-12 M • pH = 9.07

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