Buffers 1. Titrations and pH curves 2. Solubility equilibria 3. Topics in ...

ch16blank Page 3Buffer pH calculationWhat is the pH of a solution of0.10 M ammonium chloride and 0.15 M ammonia?

ch16blank Page 4Adding a strong acid or base to a bufferBuffers resist changes in pH because they contain bothan acid and a base.In a buffer of HA and A − :What happens when you add H 3 O + to the buffer?What happens when you add OH − to the buffer?

ch16blank Page 5Adding a strong acid or base to a bufferCalculate the pH when 0.0050 mol strong acid is added toa buffer which contains 0.010 mol NH 4+ and 0.015 molNH 3 ?1.Stoichiometry calculation2.Equilibrium calculation (H-H equation)

ch16blank Page 6Adding a strong acid or base to a bufferA buffer contains 0.47 M HNO 2 and 0.61 M NaNO 2 . Whatis the pH of this buffer?What is the pH when 8.7 mL of 0.128 M NaOH is addedto 243 mL of the buffer solution?

ch16blank Page 7Buffer capacity and buffer rangeRelative amounts of acid and base:A buffer is most effective when the ratio of A − to HA isbetween 0.1 and 10. What pH range does thiscorrespond to?So, when creating a buffer, make sure the pK a of theacid is as close to the desired pH as possible.What is the pH range of a buffer created by formicacid (pK a = 3.74) and its conjugate?What ratio of the base to acid would be needed tomake a pH 4.00 buffer?Absolute concentrations of acid and base:A buffer is effective when the concentrations of A −and HA are at least 10x greater than theconcentration of an added strong acid or base.

ch16blank Page 8Titrations and titration curvesKnown concentrationUnknown concentrationTitration curve:graph of pH vs volumeof solution addedStrong acid titrated with strong basepHVol NaOH added (mL)

ch16blank Page 9Strong acid titrated with strong base50.0 mL HCl of unknown concentration is titrated with0.150 M NaOH. The titration curve's only inflection pointoccurs at 40.0 mL of added base. What was the initial HClconcentration?What was the initial pH of the HCl solution?What is the pH after 60.0 mL NaOH is added?

ch16blank Page 10Weak acid titrated with strong basepHVol NaOH added (mL)HA + OH − →Buffer region:At the midpoint (half-equivalence pt):

ch16blank Page 11Weak acid titrated with strong base40.0 mL of 0.0788 M HA are titrated with 0.100 M NaOH.What is the volume of NaOH added at the equivalencepoint?If the pH = 4.88 at the midpoint, what is K a and what isthe pH at the endpoint?

ch16blank Page 12Weak acid titrated with strong baseWhat is the pH after 10.0 mL 0.100 M NaOH is added to40.0 mL of 0.0788 M HA (the titration on the last page)?

ch16blank Page 13Weak acid titrated with strong base75.0 mL of a weak acid are titrated by 0.125 M NaOH.The endpoint occurs at 62.1 mL of added base, and thepH at the midpoint is 3.12. What is the initial acid concentration? What is the K a ? What is the pH initially and at the equivalencepoint?

ch16blank Page 14Weak base and polyprotic acid titrationsWeak base titr by strong acidpHVol HCl added (mL)Polyprotic acid titr by strong basepHVol NaOH added (mL)

ch16blank Page 15pH IndicatorsMake sure the color range of your indicator is within themost vertical part of the titration curve!

ch16blank Page 16Solubility equilibriaCompounds we called "insoluble" before, are actuallyvery slightly soluble, governed by an equilibrium:CaF 2 (s) ⇌ Ca 2+ (aq) + 2 F − (aq)Solubility product constant, K sp for CaF 2 =Molar solubility, S =mol solute (when saturated)L solutionIf the molar solubility, S, of PbF 2 is 2.61 x 10 −3 M, what isthe K sp of PbF 2 ?PbF 2 (s) ⇌What are the ion concentrations?

ch16blank Page 17Calculating molar solubilityIf K sp for Mg(OH) 2 is 2.06 x 10 −13 , calculate the molarsolubility, S.

ch16blank Page 18K sp and relative solubilityDirect comparison of K sp values only works betweencompounds that dissociate into the same number ofions.Which is the most soluble?BaF 2CaF 2Fe(OH) 2PbCl 2PbBr 2K sp = 2.45 x 10 −5K sp = 1.46 x 10 −10K sp = 4.87 x 10 −17K sp = 1.17 x 10 −15K sp = 4.67 x 10 −6

ch16blank Page 19Common Ion EffectHow is solubility affected when more ions are added tothe solution?Ca(OH) 2 (s) ⇌ Ca 2+ (aq) + 2 OH − (aq); K sp = 4.68 x 10 −6To this solution, we add Ca(NO 3 ) 2Na + , K + , NH 4+ ,NO 3− , C 2 H 3 O 2−make compoundstotally solubleThe reaction shifts:A common ion in solution will _________ a compound'ssolubility.

ch16blank Page 20Common ion effect calculationWhat is S for Ca(OH) 2 ? K sp = 4.68 x 10 −6What is S for Ca(OH) 2 in 0.10 M Ca(NO 3 ) 2 solution?

ch16blank Page 21Effect of pH on solubilityIf a compound's dissociation produces a basic anion(usually OH − , CO 32−, S 2− ), solubility is dependent on pH.CaCO 3 (s) ⇌ Ca 2+ (aq) + CO 32−(aq)Addition of strong acid to this solution…A similar effect occurs when base is added to a solutionwhich contains an acidic cation.

ch16blank Page 22PrecipitationTo calculate whether or not a precipitate will form, takethe given ion concentrations and calculate Q (thereaction quotient).If Q = K sp :If Q > K sp :If Q < K sp :A newly mixed solution has these ion concentrations:[Pb 2+ ] = 0.25 M, [Cl − ] = 0.017 M. Will a precipitate form?K sp (PbCl 2 ) = 1.17 x 10 −5 .