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TITRATION

TITRATION. TITRATION. A known concentration of base (or acid) is slowly added to a solution of acid (or base). TITRATION. A pH meter or indicators are used to determine when the solution has reached the equivalence point , at which the stoichiometric amount of acid equals that of base.

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TITRATION

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  1. TITRATION

  2. TITRATION A known concentration of base (or acid) is slowly added to a solution of acid (or base).

  3. TITRATION A pH meter or indicators are used to determine when the solution has reached the equivalence point, at which the stoichiometric amount of acid equals that of base.

  4. Colors and approximate pH range of some common acid-base indicators.

  5. TITRATION OF A STRONG ACID WITH A STRONG BASE From the start of the titration to near the equivalence point, the pH goes up slowly.

  6. Titration of a Strong Acid with a Strong Base Just before and after the equivalence point, the pH increases rapidly.

  7. Titration of a Strong Acid with a Strong Base At the equivalence point, moles acid = moles base, and the solution contains only water and the salt from the cation of the base and the anion of the acid.

  8. Titration of a Strong Acid with a Strong Base As more base is added, the increase in pH again levels off.

  9. Titration of a Weak Acid with a Strong Base • Unlike in the previous case, the conjugate base of the acid affects the pH when it is formed. • The pH at the equivalence point will be >7. • Phenolphthalein is commonly used as an indicator in these titrations.

  10. Titration of a Weak Acid with a Strong Base At each point below the equivalence point, the pH of the solution during titration is determined from the amounts of the acid and its conjugate base present at that particular time.

  11. Titration of a Weak Acid with a Strong Base With weaker acids, the initial pH is higher and pH changes near the equivalence point are more subtle.

  12. Titration of a Weak Base with a Strong Acid • The pH at the equivalence point in these titrations is < 7. • Methyl red is the indicator of choice.

  13. WEAK ACID /WEAK BASE TITRATIONS

  14. Titrations of Polyprotic Acids In these cases there is an equivalence point for each dissociation.

  15. SOLUBILITY EQUILIBRIA

  16. DISSOLVING SILVER SULFATE, Ag2SO4, IN WATER • When silver sulfate dissolves it dissociates into ions. When the solution is saturated, the following equilibrium exists: Ag2SO4(s) 2 Ag+(aq) + SO42-(aq) • Since this is an equilibrium, we can write an equilibrium expression for the reaction: Ksp = [Ag+]2[SO42-] Notice that the Ag2SO4 is left out of the expression! Why? Since K is always calculated by just multiplying concentrations, it is called a “solubility product” constant - Ksp.

  17. WRITING SOLUBILITY PRODUCT EXPRESSIONS... • For each salt below, write a balanced equation showing its dissociation in water. • Then write the Ksp expression for the salt. Iron (III) hydroxide, Fe(OH)3 Nickel sulfide, NiS Silver chromate, Ag2CrO4 Zinc carbonate, ZnCO3 Calcium fluoride, CaF2

  18. SOME KspVALUES Note: These are experimentallydetermined, and may be slightly different on a different Ksp table.

  19. Calculating Ksp from solubility of a compound • A saturated solution of silver chromate, Ag2CrO4, has [Ag+] = 1.3 x 10-4 M. What is the Ksp for Ag2CrO4?

  20. Molar mass = 461.01

  21. Calculating solubility, given Ksp • The Ksp of NiCO3 is 1.4 x 10-7 at 25°C. Calculate its molar solubility. NiCO3(s) Ni2+(aq) + CO32-(aq) --- ---

  22. Molar Mass = 147.63

  23. Common Ion Effect in Solubility

  24. The Common Ion Effect on Solubility The solubility of MgF2 in pure water is 2.6 x 10-4 mol/L. What happens to the solubility if we dissolve the MgF2 in a solution of NaF, instead of pure water?

  25. Calculate the solubility of MgF2 in a solution of 0.080 M NaF. MgF2 (s)  Mg2+ (aq) + 2 F- (aq)

  26. Explaining the Common Ion Effect The presence of a common ion in a solution will lower the solubility of a salt. • LeChatelier’s Principle: The addition of the common ion will shift the solubility equilibrium backwards. This means that there is more solid salt in the solution and therefore the solubility is lower!

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