Chapter 16: Acid Base Equilibria and Solubility Equilibria Common Ion Effects Buffers Titration Solubility

Chapter 16: Common Ion Effects (1) Apply LeChatelier’s Principle. Would a common ion affect pH NH 3(aq) + H 2 O ↔ NH 4 + (aq) + OH - (aq) –Then Add the Common Ion NH 4 Cl(s)  NH 4 + (aq) + Cl - (aq)

Chapter 16: Common Ion Effects (2) What would happen to the pH of the weak acid solution given below if a common ion were added? HC 2 H 3 O 2(aq ↔ H + (aq) + C 2 H 3 O 2 - (aq) –Then Add a common Ion NaC 2 H 3 O 2  Na + (aq) + C 2 H 3 O 2 - (aq)

Sample Calculation (1) Calculate the pH of a 0.2 M solution of HC 2 H 3 O 2. Ka = 1.8E-5 (2) Addition of a common ion, C 2 H 3 O 2 -,would have which effect on the pH of the weak acid? Calculate the pH when the solution is made 0.1 M in C 2 H 3 O 2 -

Chapter 16: Buffer Solutions Definition A buffer solution maintains a relatively constant pH. Neutralizes any added/created acid. Neutralizes any added/created base.

Buffer Combinations (1)Weak Base and Salt with Common Ion : NH 3(aq) + H 2 O ↔ NH 4 + (aq) + OH - (aq) NH 4 Cl(s)  NH 4 + (aq) + Cl - (aq) (2)Weak Acid and Salt with Common Ion HC 2 H 3 O 2(aq ↔ H + (aq) + C 2 H 3 O 2 - (aq) NaC 2 H 3 O 2  Na + (aq) + C 2 H 3 O 2 - (aq)

Problem 16.12: Page 758 Which of the following solutions can act as a buffer? (a)KCN & HCN (b)Na 2 SO 4 &NaHSO 4 (c)NH 3 & NH 4 NO 3 (d)NaI & HI

Table 15.3

Buffer Calculations A 0.2M solution of HC 2 H 3 O 2 is made 0.4 M in NaC 2 H 3 O 2. Calculate the pH of this solution. Ka = 1.8E-5. HC 2 H 3 O 2(aq ↔ H + (aq) + C 2 H 3 O 2 - (aq) NaC 2 H 3 O 2  Na + (aq) + C 2 H 3 O 2 - (aq)

Buffer Calculations A 0.2M solution of HC 2 H 3 O 2 is made 0.1 M in NaC 2 H 3 O 2. Calculate the pH of this solution. Ka = 1.8E-5. HC 2 H 3 O 2(aq ↔ H + (aq) + C 2 H 3 O 2 - (aq) NaC 2 H 3 O 2  Na + (aq) + C 2 H 3 O 2 - (aq)

Henderson Hasselbach Equation Can calculate pH of a buffer from the K a or K b expression. Or could use the rearrangements of K a or K b called the Henderson Hasselbach pH = pK a + log [common ion]/[weak acid] pOH = pK b + log [common ion]/[weak base]

Problem Calculate the pH of a buffer that is 0.12 M in lactic acid (HC 3 H 5 O 3 ) and 0.11 M in sodium lactate (NaC 3 H 5 O 3 ). Ka for lactic acid = 1.4E-4 HC 3 H 5 O 3 ↔ H + (aq + C 3 H 5 O 3 - NaC 3 H 5 O 3  Na + + C 3 H 5 O 3 -

Problem How many moles of sodium hypobromite (NaBrO) should be added to 1.00 L of M hypobromous acid (HBrO) to form a buffer solution of pH of Assume that no volume change occurs when the NaBrO is added. Ka for HBrO is = 2.5E-9

Problem (a) What is the ratio of HCO 3 - to H 2 CO 3 in blood of pH 7.4? Ka for H 2 CO 3 = 4.3E-7 (b) What is the ratio of HCO 3 - to H 2 CO 3 in an exhausted marathon runner whose blood pH is 7.1? H 2 CO 3 ↔ H 3 O + + HCO 3 - EQUILIBRIUM IS CONTROLLED BY ENZYME!

Problem 16.18: Page 758 Calculate the pH of 1.00 L of the buffer 1.0 M CH 3 COONa/1.OO M CH 3 COOH before and after the addition of: Ka = 1.8E-5 (a)0.080 moles NaOH (b)0.12 moles of HCl

Chapter 16: Titration Why Do a Titration? 1. Determination of Concentration of Unknown. Titrate to equivalence point so that can calculate concentration of unknown. Moles acid = moles base indicator

Titration: Calculations at Equivalence Point 50 ml of M HCl were required to reach the equivalence point when added to 25.0 ml of NaOH. Calculate the concentration of the NaOH. Moles acid = Moles base

Acid-Base Titrations Strong Acid-Base Titrations Strong Acid-Base Titrations Consider adding a strong base (e.g. NaOH) to a solution of a strong acid (e.g. HCl).

Titration: Calculations at Equivalence Point 50 ml of M HCl were required to reach the equivalence point when added to 25.0 ml of NaOH. Calculate the concentration of the NaOH. Moles acid = Moles base

Chapter 16: Titration Why Do a Titration? 2. Measure Data for a Titration Curve Can determine Ka or Kb if have weak acid of weak base. Can also calculate the concentration of the unknown acid or base from the titration curve.

Acid-Base Titrations Strong Acid-Base Titrations Strong Acid-Base Titrations The plot of pH versus volume during a titration is a titration curve.

Acid-Base Titrations Weak Acid-Strong Base Titrations Weak Acid-Strong Base Titrations

Titration Curve: Strategy for Titration Calculations (1) Recognize that are calculating pH at a step in a titration. –Volume is constantly changing. Stoichiometry: must work with moles. –Smallest quantity will have completely reacted.

Titration Curve: Strategy for Titration Calculations (2) Calculate moles of acid and moles of base. moles = (moles/liter) (volume) moles HC 2 H 3 O 2 = moles/liter) (0.035 liters) moles NaOH = (0.150 moles/liter) ( liters)

Titration Curve: Strategy for Titration Calculations (3) If have more moles of acid than base, the acid will determine/control the pH. (4) If have more moles of base than acid, the bases will determine/control the pH. (5) The smaller quantity will completely react and disappear. acid + base  salt + water

Titration Curve: Strategy for Titration Calculations (6) If some moles of acid remain, use approach described in chapter 15 to calculate the [H + ]. –First decide if have strong acid or weak acid. –Then calculate [acid]. –[acid] = moles of acid/combined volume. –Then calculate [H + ] and the pH.

Titration Curve: Strategy for Titration Calculations (7) If some moles of base remain, use approach described in chapter 18 to calculate the [OH-]. –First decide if have strong base or weak base. –Then calculate [base]. –[base] = moles of base/combined volume

Titration Curve: Strategy for Titration Calculations (7) If some moles of base remain, use approach described in chapter 15 to calculate the [OH-]. –First decide if have strong base or weak base. –Then calculate [base]. –[base] = moles of base/combined volume

Problem A 20.0 ml sample of M HBr is titrated with M NaOH solution. Calculate the pH of the solution after the following volumes of base have been added. (b) 19.9 ml

Problem Consider the titration of 30.0 ml of M NH 3 with M HBr. Calculate the pH after the following volumes of titrant have been added. K b = 1.8E-5 (b) 10.0 ml

Problem What is the pH of a solution prepared by mixing exactly 30.0 ml of M HC 2 H 3 O 2 with 15.0 ml of M KOH.

Part B: Exp 14-B. Calculate pH of Buffer. (1) Weighed 3.5 g of NaC 2 H 3 O 2. mole= 136 g measured 2.57E-2 moles (2) Measured 8.8 ml of 3.0 M HC 2 H 3 O 2. Ka = 1.8E-5 pKa = -log (1.8E-5) = 4.74 (3) Added 55.6 ml of water. Final volume = 64.4 ml

Part B: Exp 14-B. Calculate pH of Buffer. [NaC 2 H 3 O ] = 2.57E-2 moles/ liters = M [HC 2 H 3 O ] = [3.0 moles/liter) ( liters)/ liters = M pH = pKa + log (0.462/.474) = 4.72

Addition of HCl Calculate pH of Buffer Made in 14-B After addition of 1.0 ml of 6.0 M HCl