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Acid/Base Titration and Hydrolysis of Salts

Acid/Base Titration and Hydrolysis of Salts. Last Lecture. Acid–Base Titration. The concentration of a weak acid or a weak base in water is difficult – if not impossible – to measure directly. But we can calculate the concentration from the results of titration. Acid–Base Titration.

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Acid/Base Titration and Hydrolysis of Salts

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  1. Acid/Base Titration and Hydrolysis of Salts Last Lecture

  2. Acid–Base Titration • The concentration of a weak acid or a weak base in water is difficult – if not impossible – to measure directly. • But we can calculate the concentration from the results of titration.

  3. Acid–Base Titration • A titration is a carefully controlled neutralization rxn. • To run a titration on a soln of unknown conc of an acid or a base, you need a 2nd soln called a standard soln. • A standard soln contains acid or base in a known conc

  4. Acid–Base Titration • If the unknown soln is an acid, the standard soln is a base • And vice a versa • Another substance involved in a titration is an indicator. • which is a substance that changes color at a certain pH

  5. Acid–Base Titration • Most of the time we use the common indicator Phenolphthalein. • Phenolphthalein is clear in acid • Light pink in neutral • Dark pink in base

  6. Procedure for Titration • the standard is slowly added to the unknown soln • the 2 solns mix, the acid in one neutralizes the base in the other • Eventually, enough standard soln is added to neutralize all of the acid or base in the unknown soln.

  7. 1 2 4 3

  8. Procedure for Titration • The point at which this occurs is called the equivalence point. • [H30+] = [OH-] • The point at which the indicator changes color is called the end point of the titration.

  9. Procedure for Titration • If the indicator is chosen correctly the end point is very close to the equivalence point. • Therefore, at approximately the end pt of a titration the total mols of H+ donated by the acid is equal to the total mols of H+ accepted by the base.

  10. Total moles of H+ from the acid Total moles of OH- from the base = Procedure for Titration • For Arrhenius Acids and Bases we can say: • This is key to solving titration calcs

  11. AH A-

  12. AH A-

  13. AH A-

  14. AH A-

  15. AH A-

  16. AH A-

  17. A- AH

  18. A- AH

  19. A- AH

  20. A- AH

  21. A- AH

  22. Sample Problem Solns of NaOH are used to unclog drains. A 43 ml volume of NaOH was titrated with 32 ml of .100M HCl. What is the molarity of the NaOH soln?

  23. Analyze: • At the equivalence pt, the added acid has completely neutralized the unknown base. • It took 32 ml of acid to neutralize 43 ml of base • We know that mols of base = mols of acid, at the equiv pt

  24. Solve: • moles of acid used: • (.100mols/L)(.032L)= .0032mol of acid • mols of acid = mols of base • .0032 mol base used in rxn. • Molarity of NaOH • .0032mol/.043L = .074M

  25. Hydrolysis Reactions • An interesting phenomena when discussing acids and bases, is that some salts when dissolved in water produce solutions that are acidic or basic • One way to predict the behavior of a salt in water is to follow the approach taken in many text books.

  26. Hydrolysis Reactions • A salt may be thought of as the product of a rxn between an acid and a base • When the parent acid (PA) and base (PB) are strong, a water soln of the salt is neutral

  27. Hydrolysis Reactions • When a salt is formed from the rxn between a weak acid (WA) and a strong base (SB), a water soln will be basic. • When a salt is formed from a strong acid(SA) and a weak base(WB), a H2O soln of the salt will be acidic

  28. Hydrolysis Reactions • When a salt is formed from a WA and a WB, a water soln of the salt could be acidic, basic, or neutral depending on the nature of the ions • The reaction of ions with water leading to an alteration of the pH of a water soln called Hydrolysis

  29. HCl NaCl+HOH +NaOH PA PB H2SO4 +NH3 (NH4)2SO4+H2O PA PB Parent acids/bases • Given a salt we can think backwards to determine its parent acid/ and parent base

  30. Parent acids/bases • Remember that salts dissolved in water dissociate into ions • Break up into the ions that make up the salt • Remember that some salts can then react with water to make the solution acidic or basic

  31. HCl  Cl- OH- H3O+ Parent acids/bases • How do we know if the salt will react with water or it won’t? • First we need some rules • Rule #1 • Strong PAs/Strong PBs form weak CBs/weak Cas SPA WCB SPB WCA

  32. H2CO3HCO3- NH3NH4+ Parent acids/bases • Weak PAs/Weak PBs react with water to form SBs/SAs WPA SCB WPB SCA • Rule #2: • -The strong CAs/strong CBs of weak PAs and weak PB WILL REACT with water

  33. Parent acids/bases • The WCAs/WCBs of SPAs and SPBs DO NOT REACT with water • So what does it all mean? • Let’s put it all together -

  34. acidic

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