Supplemental Instruction 1060 Hixson-Lied Student Success Center  294-6624  www.si.iastate.edu Chapter 17 Additional Aspects of Aqueous Equilibria Supplemental Instruction Iowa State University Leader: Alarm Cho Course: Chem 178 Instructor: John Verkade Date: Oct. 18, 2009 pH Range: pH range over which the buffer acts effectively . Buffers most effectively resist a change in pH in either direction when the concentrations of weak acid and conjugate base are about the same. From the Henderson-Hasselbalch EQ., when [HA]=[A-], This gives the optimal pH of any buffer. Strong Acids/Strong Bases added to Buffer - Problem solving strategies 1. Acid-Base neutralization reaction and its effect on the [HA] and [A-]. This step involves stoichiometric calculations. 2. Use Ka and new concentrations of [HA] and [A-] from step1 to calculate pH using the H-H EQ. 3. Then find [H+] if needed. - Assumptions: Strong acids and Strong bases dissociate completely. Calculating pH changes in Buffers A buffer is made by adding 0.300 mol HC2H3O2 and 0.300 mol NaC2H3O2 to enough water to make 1.00 L of solution. The pH of the buffer is 4.74. Calculate the pH of this solution after 0.020 mol of NaOH is added. - Stoichiometric calculation ICE table CH3COOH OH- H2O CH3COO- Initial 0.300 0 - 0.300 Change - 0.020 - Equilibrium 0.280 0 - 0.320- Equilibrium calculation Use H-H EQ to calculate pH. we know that when [HA]=[A-], the pH=pKa. So the equation will look like, Titrations - Definition: technique used to determine the concentration of an unknown solution by adding a standard solution of known concentration. - Equivalence point: When the stoichiometric equivalence is reached between the acid/base in the unknown solution and the acid/base of the standard solution. - End point: point of color change in the indicator. Phenolphthalein is commonly used for weak acid-strong base titrations. - Three cases: 1. Strong acid- Strong base a. Initial pH The pH of the solution is determined by initial concentration of acid. b. Between initial pH and equivalence point pH increases slowly as strong base is added and then increases rapidly in the vicinity of the equivalence point. pH is determined by the amount of acid that has not been neutralized in the solution. c. Equivalence point An equal number of moles of strong base and strong acid have reacted. The pH of solution is 7.00 since the remaining salts don’t hydrolyze. d. After equivalence point pH is determined by the excess amount of base in the solution. 2. Weak Acid- Strong base a. Initial pH We use the Ka of the acid to calculate pH. b. Between initial pH and equivalence point The calculations for pH in this step is identical to the procedure for calculating pH when adding a strong acid/base to buffers. The solution contains a mixture of the weak acid and its conjugate base. We need to know the amount of OH- from the strong base that neutralized the weak acid and the changes in the concentrations of the weak acid and its conjugate base. c. Equivalence point Once all the weak acid has been neutralized with the strong base, the remaining compound is the salt of the conjugate base. Thus, the pH will be higher than 7.00. d. After equivalence point pH is determined by the excess amount of base in the solution. - For weaker Acids, the initial pH is higher and the pH change near the equivalence point is more subtle. - Shape of the weak base-strong acid titration curve is reverse of that of the weak acid-strong base titration curve. - Polyprotic acids have multiple equivalence points corresponding to the number of protons being released.Solubility Product - Definition: The equilibrium constant for the equilibrium between an ionic solid and its saturated solution. BaSO4(s) Ba2+(aq) + SO42-(aq) - Denoted as Ksp, - Units: unitless Solubility - Definition: quantity that dissolves to form a saturated solution. - Units: g/L or moles/L (molar