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Acids – Bases Equilibria Part IV: Analysis of Salts

Acids – Bases Equilibria Part IV: Analysis of Salts. Jespersen Chap. 17 Sec 6. Dr. C. Yau Spring 2014. SALT SOLUTIONS HCl + NaOH H 2 O + NaCl Acid base water salt We say that HCl has been neutralized by NaOH. The product mixture is neutral.

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Acids – Bases Equilibria Part IV: Analysis of Salts

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  1. Acids – Bases EquilibriaPart IV: Analysis of Salts Jespersen Chap. 17 Sec 6 Dr. C. Yau Spring 2014

  2. SALT SOLUTIONS HCl + NaOH H2O + NaCl Acid base water salt We say that HCl has been neutralized by NaOH. The product mixture is neutral. HCl + NH4OH H2O + NH4Cl acid base water salt The product mixture is acidic! Why? NH4Cl dissociates into NH4+ and Cl- and NH4+ is slightly acidic! NH4+ + H2O NH3 + H3O+ weak base Conj. acid of weak base Is a “strong” acid.

  3. In the same way, HC2H3O2 + NaOH H2O + NaC2H3O2 The product mixture is not neutral. NaC2H3O2 dissociates into Na+ and C2H3O2-and acetate is the conjugate base of a weak acid (acetic acid). C2H3O2- is a relatively strong base. It will pull a proton off water: C2H3O2- + HOH HC2H3O2 + OH- producing OH- ions. THUS SODIUM ACETATE PRODUCES A SLIGHTLY BASIC SOLUTION!

  4. Predicting Acid–Base Properties Of A Salt 1. If neither the cation nor the anion can affect the pH, the solution should be neutral. 2. If only the cation of the salt is acidic, the solution will be acidic. 3. If only the anion of the salt is basic, the solution will be basic. 4. If a salt has a cation that is acidic and an anion that is basic, the pH of the solution is determined by the relative strengths of the acid and base. Compare Ka and Kb for the ions Cation may be acidic. Anion may be basic.

  5. Predicting Effects Of Ions On pH Most metal cations are neutral or extremely weak acids (e.g. Na+, Ca2+) Conjugate acids of weak bases are acidic (e.g. NH3, N2H4 NH4+, N2H5+) Conjugate bases of weak acids are basic (e.g. HC2H3O2, HNO2  C2H3O2¯, NO2¯) Conjugate bases of the strong acids are neutral (except HSO4- which is still fairly acidic) (e.g. HCl, HNO3  Cl¯, NO3¯) but not H2SO4 HSO4¯ Still acidic!

  6. 0.1M solutions of the following are acidic basic/ neutral or amphoteric? … NaCl NaCN NH4Cl HCN NaClO4 HCl Na3PO4 • neutral • basic • acidic • acidic • neutral • acidic • basic Do p. 795: Pract Exer 25 & 26.

  7. At this point you are expected to recognize whether a salt will produce an acidic or basic solution from looking at its formula. That is a qualitativeanalysis. We now examine how to analyze the pH of salt solutions quantitatively. Decide whether cation or anion will affect pH. Write eqn for the Ka or Kb of ion. Calc Ka or Kb from Kb or Ka of its conjugate acid/base Set up ICE table. CalcpH.

  8. Example 17.5 p. 796 What is the pH of a 0.10 M solution of NaOCl? What do we look up? Need Kb of OCl-. So find Ka of HOCl = 3.0x10-8 Then what? Set up ICE table for what? Example What is the pH of a 0.20 M solution of hydrazinium chloride N2H5Cl? What do we look up? Hydrazine N2H4 is a weak base with Kb = 9.6x10-7 Do Pract Exer 27, 28 & 29 p. 797

  9. Example Will an aqueous solution that is 0.20 M NH4F be acidic, basic or neutral? Kb of NH3 = 1.8x10-5 Ka of HF = 3.5x10-4 ...and so? Do Pract Exer 30 & 31 p.798

  10. Summary of what we did in the previous examples Example 17.5: Calculation for pH of salt made from strong base and weak acid (NaOCl). Example with N2H5Cl. : Calculation for pH of salt made from weak base and strong acid ExampleNH4F: Analysis of salt made from weak base and weak acid.

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