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posted by  The VC on 3/9/2007 12:32:50 PM  |  status: Live  

Salt Solutions

Course Textbook Chapter Problem
General Chemistry Chemistry Principles and Reactions Updated 5th Edition 13 Review Section 64
Question Details:
Hi, I'm having trouble with the following problem.  I'm not sure how I can calculate pH without knowing the Ka values for the following compounds.

64.  Arrange the following 0.1 M aqueous solutions in order of increasing pH (lowest to highest).

KOH  ,  KF  ,  KCl  ,  ZnCl2  ,  HCl

I will greatly appreciate any help I can get.

Sincerely,

Andrew
The VC
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AAnswers:

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Scholar
Karma Points: 368
posted by Jfcap on 3/10/2007 10:27:37 AM  |  status: Live
Asker's Rating: Lifesaver   
The VC's comment:
"I like the table provided. It helps put these compounds in perspective!"
Response Details:
This chart is from my chemistry class, it should help.  You do not need the Ka/Kb values on the chart to answer the question.  The chart starts with strong acids at the top (and its conjugate base, which is NOT a base), and as you go down the chart, acids get weaker, and the conj. base gets stronger.

Acid-Conjugate Base Ka-Kb Table

 

Ka of acid

Formula of acid

Formula of conj. base

Kb of conj. Base

humungous

HClO4

ClO4-

<<1.0 x 10-14

humungous

H2SO4

HSO4-

<<1.0 x 10-14

humungous

HCl

Cl-

<<1.0 x 10-14

humungous

HNO3

NO3-

<<1.0 x 10-14

 

H3O+

H2O

1.0 x 10-14

1.2 x 10-2

H2SO3

HSO3-

8.3 x 10-13

7.5 x 10-3

H3PO4

H2PO4-

1.3 x 10-12

2.1 x 10-4

HCO2H

HCO2-

4.8 x 10-11

7.2 x 10-4

HF

F-

1.4 x 10-11

1.8 x 10-5

HCH3CO2

CH3CO2-

5.6 x 10-10

4.2 x 10-7

H2CO3

HCO3-

2.4 x 10-8

1 x 10-7

H2S

HS-

1 x 10-7

6.3 x 10-8

H2PO4-

HPO42-

1.6 x 10-7

5.6 x 10-10

NH4+

NH3

1.8 x 10-5

4.0 x 10-10

HCN

CN-

2.5 x 10-5

4.9 x 10-11

HCO3-

CO32-

2.1 x 10-4

4.2 x 10-13

HPO42-

PO43-

2.4 x 10-2

1.0 x 10-14

H2O

OH-

 

<<1.0 x 10-14

C2H5OH

C2H5O-

humungous

<<1.0 x 10-14

CH3OH

CH3O-

humungous

<<1.0 x 10-14

H2

H-

humungous

<<1.0 x 10-14

CH4

CH3-

humungous

 

 

 

 

 

 

 

 

 In order from low pH to high pH, it should be: KOH, KF, KCl, ZnCl2 and then HCl.


I hope that helps =)



 
(Cramster SME)
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posted by christine(cramster SME) on 3/15/2007 12:56:49 AM  |  status: Live
Asker's Rating: Lifesaver   
The VC's comment:
"Good explanation using the theory of Acids and Bases! This helps in case I don't have the table!"
Response Details:
 
 Remember one thing u will always not have the table with u, so there is another simpler method of finding the pH strenth
 We all know that HCl is a strong acid thus its pH will be lowest among all the given compounds and KOH is a strong base whose pH will be highest among all .
 So to write the given components in order of increasing pH
 HCl ----------------------------KOH
 Now we have to fix the other compounds in between these two.
 Lets take KCl , cation K+ is the conjugate acid of strong base KOH will not affect the pH of the solution and Cl- anion is the conjugate base of strong acid HCl will also not affect the pH of the solution, thus the combined effect is pH neutral.
 About KF , cation is the conjugate acid of strong base KOH will not affect the pH and F-is the conjugate base of weak will cause an increase in pH thus the combined effect is a basic solution.
 For ZnCl2, anion Cl- is the conjugate base of strong acod HCl will not affect the pH but the cation Zn2+ is the conjuate acid of a weak base will cause decrease in pH which means the combined effect is an acidic solution.
 Now we arrange them all in the order of increasing pH means from lower pH to higher
  HCl< ZnCl2 < KCl< KF<KOH
    Hope this helps!
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