L29 p HSalts Problems forstudents PDF

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L29 p HSalts Problems...

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CHE 132: General Chemistry II Chpt 17: pH of salts Lecture 29 Dr. Nagan Clicker Channel: CHE132S21

Office Hrs: MWF 11:23-11:55 AM, MWF 3:35-4:00 PM [email protected]

Salts that Produce Neutral Solutions Cations of strong bases and anions of strong acids have no effect on pH

Neutral Cations Group I: Li+, Na+, K+, Rb+,Cs+ Group II: Mg2+, Ca2+, Sr2+, Ba2+ Neutral Anions Cl—, Br—, I—, NO3—, ClO4—, ClO3— Note: HSO4— is a weak acid

Ex: NaCl(s) dissolved in water

Na+ - pH neutral Cl- - pH neutral NaCl(aq) – neutral solution

Acid-Base Properties of Salts Salts that Produce Basic Solutions Consider NaF (aq):

Na+

F—

H2O

• Na+ is neutral • F— is conjugate base of HF, so weak base • H2O can be acid or base

F—(aq) + H2O (l) weak base

OH—(aq) + HF(aq)

acid

• Kb equation dominates • Basic solution • Use Kb to determine pH

Acid-Base Properties of Salts Salts that Produce Acidic Solutions Consider NH4Cl (aq):

NH4+

Cl—

H2O

• Cl— is neutral • NH4+ is conjugate acid of NH3, so weak acid • H2O can be acid or base

NH4 +(aq) + H2O (l) weak acid

H3O+(aq) + NH3(aq)

base

• Ka equation dominates • Acidic solution • Use Ka to determine pH

Acid-Base Properties of Salts Salts where you have to decide if Ka or Kb dominates Consider NH4CN (aq):

NH4+

CN—

H2O

• CN— is basic • NH4+ is weak acid • H2O can be acid or base

Kb (CN—) = 1.6 x 10-5

Ka (NH4+) = 5.6 x 10-10

Because Kb>Ka, • solution is basic • use Kb equation to determine pH

Acid-Base Properties of Salts Salts where you have to decide if Ka or Kb dominates In general

• if Kb>Ka, basic solution, use Kb for pH • if Ka>Kb, acidic solution, use Ka for pH • if Ka=Kb, neutral solution

Summary: Salts Divide salt into cation and anion (M+ and X—)

Is the cation acidic or neutral?

Is the anion basic or neutral? weird exception

Anion of a polyprotic acid? Look at Ka vs. Kb

Group I or II cation? neutral Conjugate acid of a weak base? acidic From strong acid? neutral Conjugate base of a weak acid? basic

Problem: Consider the following data on some weak acids and weak bases: HNO2 Ka = 4.5 x 10-4 HONH2 Kb = 1.1 x 10-8

HCN Ka = 4.9 x 10-10 C2H5NH2 Kb = 6.4 x 10-4

Rank the following salts in order of increasing pH.: 0.1 M HONH3Br

0.1 M KNO3 0.1 M C2H5NH3Cl

0.1 M NaCN

Problem: Calculate the pH of a 0.55 M solution of lidocaine HCl (C14H21NONH2Cl). Note that lidocaine (C14H21NONH) is a weak base with a pKb = 7.94. Strategy: • categorize cation and anion (acidic, basic, neutral) • determine pH Calc Kb pKb=–log Kb Kb=10-pK

pOH + pH = 14.00 pH = 14.00 - pOH

b

Categorize cation and anion into acidic/basic/neutral

If need be compare Kb and Ka

Calc Kb pKb=–log Kb Kb=10-pK b

pOH = —log [OH—]

basic?

Do ICE Table acidic?

Ka=Kw/Kb

pH = —log [H+]...