Chapter 7 Mixtures of Acids and Bases Common Ion effect Buffer solutions Behavior of a solution in which the same ion is produced by two different compounds a solution of weak acid and soluble ionic salt of the weak acid b solution of weak base and soluble ionic salt of the weak base Types 1 Calculate the H3O and pH of a 0 10 M in acetic acid HCH3CO2 and 0 20 M in NaCH3CO2 Ka 1 8 x 10 5 2 Will more lead chloride dissolve in pure water or in 0 1 M NaCl solution Given PbCl2 s Pb2 aq 2Cl1 aq K 1 7 x 10 5 Common ion effect demonstrates Le Chatelier s Principle system responds to the addition of an ion Treatment of buffers is the same as common ion effect 3 What is the pH of 250 0 mL of 0 168 M NH3 Kb 1 8 x 10 5 4 What is the pH of a mixture of 250 0 mL of 0 168 M NH3 and 20 0 mL of 3 50 M NaOH Kb 1 8 x 10 5 Buffersa solution of a weak acid HA and its conjugate base A1 in comparable and appreciable amounts ratios of 10 1 5 Calculate the pH of a solution that is 0 3 M HOCl and 0 5 M NaOCl KaHOCl 3 5 x 10 8 Henderson Hasselbalch Equation pH pKa log A1 HA 6 Calculate the pH of a solution that is 0 3 M HOCl and 0 5 M NaOCl pKa 7 46 SAME RESULT K EXPRESSION and HH EQUATION Buffers solution resists changes in pH upon addition of strong acid or base Since a weak base is present A1 additional H3O1 Since a weak acid is present HA additional OH1 Buffer Capacity can be absorbed A1 H3O1 HA H2O can be absorbed HA OH1 A1 H2O pH pKa log A1 HA pH pKa pKa 1 7 A solution buffered at 5 00 is needed in a chemistry experiment Can we use acetic acid HC2H3O2 and sodium acetate NaC2H3O2 to make it If so what ratio is needed 8 A buffer solution was prepared by dissolving 2 5 g of NH4Cl in 125 ml of 0 24 M NH3 At what pH will this solution serve as a buffer 9 Calculate the H in a buffer solution prepared by mixing 200 0 ml of 0 10 M NaF and 100 ml of 0 050 M HF KHF 7 2 x 10 4 10 Calculate the grams of NH4Cl that must be used to prepare 500 0 ml that is 0 10 M in aqueous ammonia and has a pH of 9 15 11 Calculate the ratio of CO3 buffering at pH 9 50 The pKa2 of carbonic acid H2CO3 is 10 33 1 ions needed to achieve 2 and HCO3 Typical buffer systems Composition pKa Composition pKa Acid buffers CH3CO2H CH3CO2 HNO2 NO2 HClO2 ClO2 4 74 3 37 2 00 Basic buffers NH3 NH4 H2PO4 HPO4 9 26 7 21 Treatment of buffer solutions is the SAME as common ion effect The following problems have a buffer system established and acid or base added to disrupt it 12 a Calculate the pH of a buffer solution that is 0 040 M Na2HPO4 and 0 080 M KH2PO4 at 25 C KH2PO4 6 2 x 10 8 b Calculate the resulting pH of this buffer solution when 1 2 g of NaOH are dissolved in 500 ml of the above solution c Calculate the pH when 1 2 g of NaOH is dissolved in pure water 13 a A 1 liter buffer solution is 0 10 M in both acetic acid and sodium acetate How much will the pH change when 0 010 mole of NaOH is added b What if 0 010 mole of HCl were added instead of the NaOH Recapping Mixtures of Solutions 1 Weak Acid Weak Base BUFFERS K depends on the K s of the reacting species Whichever is stronger dictates the pH 2 Strong Acid Strong Base H3O1 OH1 2 H2O 3 Strong Acid Weak Base A1 H3O1 HA H2O 4 Weak Acid Strong Base HA OH1 A1 H2O These react ly complete A buffer may be formed Section 7 3 THE strongest acid that can exist in water is H3O THE strongest base that can exist in water is OH 14 If 51 0 mL of 0 200 M HCl are mixed with 29 0 mL of 0 140 M NaOH what is the pH of the final solution 15 If 32 0 mL of 0 321 M HCl are mixed with 28 0 mL of 0 222 M NaF what is the pH of the final solution pKa HF 3 14 16 If 11 0 mL of 0 175 M HCl are mixed with 44 0 mL of 0 361 M NaF what is the pH of the final solution pKa HF 3 14 17 If 15 0 mL of 0 200 M HCl are mixed with 30 0 mL of 0 100 M NaF what is the pH of the final solution pKa HF 3 14 Acid base titrations are covered in CH 202 labs You may read about titration curves in sections 7 3 5 to 7 3 20 However in this course only acid base reactions will be tested No titration curves will be part of your examinations