Ionic Strength and pH: Concepts, Calculations, and Examples - Prof. Abdo, Lecture notes of Analytical Chemistry

Download Ionic Strength and pH: Concepts, Calculations, and Examples - Prof. Abdo and more Lecture notes Analytical Chemistry in PDF only on Docsity!

Chapter 3

Activity

Ionic strength

Where,

ci is the concentration of the species

zi is its charge of the species.

 

 

i

ci zi

c z c z

2

2 2 2

2 1 1

Example: Calculation of Ionic Strength

Find the ionic strength of

(a) 0.10 M NaNO3 ;

Solution:

(a)

  (^)     (^)   (^)      

2 3

2

Na. NO.

 

1. 10 M

2. 10. 1 + 0.10. 1 2

1





Find the ionic strength of

(c) 0.020 M KBr + 0.010 M Na 2 SO

  (^)  ^    (^)   (^)  ^    (^)   (^)  ^    (^)   (^)       1 2

K. 1 2 Br. 1 2 Na. 1 2 SO 42. 2 2

       

1. 050 M

2. 20. 1 + 0.020. 1 + 0. 020. 1 + 0.010. 4 2

1





(c)

 NaNO 3 is called a 1:1 electrolyte because the

cation and the anion both have a charge of 1.

 For 1:1 electrolytes , the ionic strength equals

the molarity.

 For any other stoichiometry (such as the 2:

electrolyte Na 2 SO 4 ).

The ionic strength is greater than the molarity.

Activity Coefficientsمعامل الفاعلية

Activity of C: AC = [C] C

Activity Concentration Activity coefficient

of C of C o f C

The activity coefficient

(measures the deviation of behavior from ideality)

If the activity coefficient is =1 ,

then the behavior would be ideal. (^) 10

The correct form of the equilibrium constant is:

General form of equilibrium constants:

   

   

K

A A

A A

C D

A B

C

c D

d

A

a B

b

c C

c d D

d

a A

a b B

b

 

 

 

Equation ( # )allows for the effect of ionic strengh

on a chemical equilibrium because the activity

coefficients depend on ionic strengh.

(#)

pH Revisited

The definition of pH given, pH = - log [H+], is not exact,

the real definition is:

When we measure pH with a pH- meter, we are

measuring the negative logarithm of the hydrogen ion activity, not its concentration.

pH A H H H

  (^)    

 log log[ ]

Activity

Example: pH of Pure Water at 25°C

Calculate the pH of pure water by using activity coefficient correctly. Solution: The relevant عالقة equilibrium is:

Stoichiometry tells us that H+ and OH ^ are produced in a 1:1 mole ratio,

H 2 O H+^ + OH

Kw

K (^) w  A (^) H   A (^) OH   [ H ^ ]  (^) H  [ OH]OH

K (^) w  10. x 10-14^  ( )x  (^) H  ( )x OH

1.0 x 10-14^ = x^2  x = 1.0 x 10-7^ M

7. 00

log ( 1. 0 10 ) ( 1. 00 )

log[ ] 7 

 

  

 (^) 

x

pH H  H

The concentration of H+^ and OH ^ are equal and are both greater than 1.0 x 10-7^ M.

The activities of H+^ and OH ^ are not equal in this solution:

Finally, we calculate the pH:

A (^) H   [ H^ ]  (^) H  ( .126 x 10 ^7 ) (. 083 )  105. x 10 ^7

A (^) H   [OH ^ ]  (^) OH  ( .126 x 10 ^7 ) ( .0 76 ) 0 96 x 10. ^7

log ( 1. 05 10 ) 6. 98

log

 ^7 

  



x

pH AH

Thank You