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Acid/Base Titration

Acid/Base Titration. Technique used to determine the concentration of an acid or base by comparison with a standard. A neutralization reaction is carried out. Rely on color change to signal end of reaction. Titration: Monoprotic Acid + Monohydroxy Base. HA + MOH  MA + H 2 O

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Acid/Base Titration

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  1. Acid/Base Titration • Technique used to determine the concentration of an acid or base by comparison with a standard. • A neutralization reaction is carried out. • Rely on color change to signal end of reaction.

  2. Titration: Monoprotic Acid + Monohydroxy Base • HA + MOH  MA + H2O • Use the relationship in the balanced equation to find the unknown concentration. • At the end-point, # moles HA = # moles MOH

  3. How to calculate # of moles? • Remember Molarity! • Molarity = # moles solute • Rearranging: # moles solute = Liters of solution Molarity X Liters of solution

  4. Acid-Base Titration • Carefully controlled neutralization reaction. • Requires a standard solution and an acid-base indicator. • Standard solution has an acid or base of known concentration.

  5. Titration • Standard solution is slowly added to unknown solution. • As the solutions mix, a neutralization reaction occurs. • Eventually, enough standard solution is added to neutralize the unknown solution.  Equivalence point. Simple Titration Animation

  6. Equivalence point • Total number of moles of H+ ions donated by acid = total number of moles of H+ accepted by base. • Total moles H+ = total moles OH-

  7. Titration • End-point = point at which indicator changes color. • If the indicator is chosen correctly, the end-point is very close to the equivalence point.

  8. Titration of a strong acid with a strong base 14- Phenolphthalein Color change: 8.2 to 10  pH Between pH of 4 and 10, only a few drops of base are added. 7- Equivalence Pt  0-    Volume of 0.100 M NaOH added (ml) 0 ml 40ml 20 ml

  9. MH+VH+ = MOH-VOH- • MH+ = molarity of H+ • MOH- = molarity of OH- • VH+ = volume of H+ • VOH- = volume of OH- • If you know 3, you can find the 4th.

  10. MaVa = MbVb • True for monoprotic acids and monohydroxy bases.

  11. Titration Problem #1 • In a titration of 40.0 mL of a nitric acid solution, the end point is reached when 35.0 mL of 0.100 M NaOH is added. Calculate the concentration of the nitric acid solution.

  12. Neutralization Reaction • HNO3 + NaOH  H2O + NaNO3 • 1 H in acid & 1 OH in base. • MaVa = MbVb

  13. Variables • Ma = ? • Va = 40.0 mL • Mb = 0.100 M • Vb = 35.0 mL

  14. Plug and Chug • X (40.0 mL) = (0.100 M )(35.0 mL) • X = .0875 M

  15. Titration Problem #2 • What is the concentration of a hydrochloric acid solution if 50.0 mL of a 0.250 M KOH solution are needed to neutralize 20.0 mL of the HCl solution of unknown concentration?

  16. Neutralization Reaction • KOH + HCl  H2O + KCl • 1 H in acid & 1 OH in base • MaVa = MbVb

  17. Variables • Ma = X • Va = 20.0 mL • Mb = 0.250 M • Vb = 50.0 mL

  18. Plug and Chug • X (20.0 mL) = (0.250 M) (50.0 mL) • X = 0.625 M

  19. Titration Problem #3 • What is the concentration of a sulfuric acid solution if 50. mL of a 0.25 M Ba(OH)2 solution are needed to neutralize 20 mL of the H2SO4 solution of unknown concentration?

  20. Neutralization Reaction • H2SO4 + Ba(OH)2 2 H2O + BaSO4 • H2SO4 is a strong diprotic acid. • Ba(OH)2 is a strong dihydroxy base. • # of H’s in acid = # of OH’s in base. • MaVa = MbVb

  21. Variables • Ma = X • Va = 20. mL • Mb = 0.25 M • Vb = 50. mL

  22. Plug and Chug • X (20.) = (0.25 M) (50. mL) • X = 0.625 M = MH+ • To calculate the Molarity of the H2SO4, adjust for the fact that the acid is diprotic.

  23. [H+] vs. [H2SO4] • H2SO4 2H+ + SO42- • Molarity of H2SO4 = ½ [H+] • ½ (0.625 M) = 0.31 M H2SO4

  24. Other kinds of Titrations • Titrations based on redox reactions are also used. • Same ideas: rely on color change to signal completion of reaction. • Use stoichiometric equalities to determine concentration of unknown. Redox titration Animation

  25. Salts • A salt is an ionic compound containing positive ions other than H+ and negative ions other than OH- • Hydrolysis is the opposite of neutralization. • Literally means “water splitting.” • A reaction between water and the ions of a dissolved salt. • When a salt undergoes hydrolysis, it reacts with water to form an acid and a base. Acid + Base  Salt + Water

  26. Hydrolysis • To predict which solution occurs when salts react with water, i.e., neutral, acidic or basic: • Identify the cation and anion present in the salt • Determine if from a strong or a weak acid/base • General Rule: strong acid + strong base  neutral salt strong acid + weak base  acidic salt weak acid + strong base  basic salt

  27. Buffers • Buffers are solutions that resist changes in pH when limited amounts of acid or base are added. • A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. • The mixture of ions and molecules resist changes in pH by reacting with any H+ or OH- added to the solution.

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