DISSOCIATION OF WATER

One of the most important properties of water is its ability to act both as an acid and as a base. In presence of an acid, water acts as a base where as in the presence of base, water acts as an acid.

Acid + Base     < ----- > Acid + Base

HCl (aq) + H2O (l) < ----- > H3O+ (aq) + Cl-(aq)

H2O (l) + NH3 (aq) < ----- > NH+4 (aq) + OH-(aq)

A substance (such as water) that can behave as both an acid and a base is said to be an amphoteric substance. The amphoteric nature of water is best seen in its self ionization. A proton from one water molecule is transferred to another water molecule, leaving behind (OH-) ion and forming (H3O+) ion.

9.5.1 The Ion Product of Water:

In the study of acid-base reaction in aqueous solution, the important quantity is the (H+) ions concentration, expressing the proton as (H+) rather than (H3O+), we can write the equilibrium constant (Kc) for the auto ionization water.

H2O < ----- > H+ (aq) + OH-(aq)

Kc= [H+] [OH-]/H2O

Since a very small fraction of water molecules are ionized, the concentration of water, that is [H2O] remains mostly unchanged, therefore,

K [H2O] = Kw = [H+] [OH-]

The equation constant (Kw) is called the ion-product constant, which is the product of molar concentration of (H+) and (OH-) ions at a particular temperature. In pure water at 25G, the concentration of H+ and OH- ions is equal and found to be:       

                   [H+] = 1x10-7 M and [OH-]   =      1x10-7 M

Thus             Kw = (1x10-7) x (1x10-7)      =       21x10-14

 

Whenever [H+] = [OH-], the aqueous solution is said to be neutral, if the number of (H+)ions increase, the aqueous solution is acidic. When the number of (OH-) ions increase, the aqueous solution is basic.

For example, the (H+) ion concentration of any solution is 1x10-4, and then the (OH-) ion concentration must change to,    

Kw      =                 [H+] [OH-]     = 1x10-14

[OH-]   =                 Kw / [H+]       = 1x10-14/1x10-4

=                 1x10-14x104

=                 1x10-10M

(OH-) ion concentration = 1x10-10M

9.5.2 The Concept of (pH):

It is well known fact that the strength of acidic solution is measured in males per litre of a solution. A one molar (1M) solution of HCl contains 1 mole of (H3O+) ions in each litre (dm3) of solution. In pure water (or any neutral solution) there are 1x10-7M (moles per litre) of H3O+ ions and equal number i.e. (1x10-7M) of OH- ions. To avoid the use of complex numbers, such as 1x10-7M or 1x10-10M. To express the concentration of H+ and OH- ions, it is convenient to express the acidity or basicity of a solution in terms of pH. In 1909 the Danish Chemist S.P.L. Sorensen proposed that only the number in the exponent be used to express the acidity, called (pH) from the French (Pouvoir hydrogene=hydrogen power). On this scale, a concentration of (1x10-7) moles of H3O+ ions per litre of solution becomes a pH of 7. Similarly a concentration 1x10-10 M becomes a pH of 10, and so on. Thus (pH) of a solution is defined as the negative logarithm of the hydrogen ion (H+) concentration or (H3O+) ion concentration (in moles per litre).

Mathematically, we can write pH = -log [H+]

Thus, a pure water (or any neutral solution) in which (H+) ion concentration is 1x10-7 M, has a pH of 7.

 

Since            pH =   -log [H+]

pH =   -log [10-7]

pH =   -[-7]

pH =   7

Similarly pOH is the negative logarithm of hydroxide ion (OH-) concentration.

Mathematically, we can write pOH = -log [OH-]

The sum of pH and pOH of a solution is always equal to 14.

i.e. PH + pOH = 14

 

Hydrogen and Hydroxide Ion Concentration in Various Acidic and Basic Water Solutions at 25C

Degree of acidity

[H+]

 

[OH-]

 

Preparation

pH

pOH

Very acidic

10-1

10-0

10-1

10-15

10-14

10-13

 

-1

0

1

15

14

13

Medium acidic

10-2

10-3

10-4

10-12

10-11

10-10

Add sufficient acid of water

2

3

4

12

11

10

Slightly acidic

10-5

10-6

10-9

10-8

 

5

6

9

8

Neutral

10-7

10-7

Pure water of solution of neutral substance

7

7

 

Slightly basic

10-8

10-9

10-6

10-5

 

8

9

6

5

Medium alkalinity

10-10

10-11

10-12

10-4

10-3

10-2

 

10

11

12

4

3

2

Very basic

10-13

10-14

10-15

10-1

10-0

10-1

Add sufficient base of water

13

14

15

 

1

0

-1

 

The following examples illustrate the calculation of pH, pOH and (H+) ion

Concentrations.

 

Examples. (1) Calculate the pH of 0.01 M HCL solution?

Solution:

We know that, HCl is strong acid, so it is completely ionized in solution.

Then pH = - log [H+]

Express [H+] in exponential form

0.01   M ------à  1x10-2M

pH      =       - log[H+]  

pH      =       - log[10-2]

=       - log[-2]

pH      =       2 (Answer)

Example. (2) Calculate the pH of a solution whose (H+)ion concentration is

5x10-4M?

Solution

pH      =       -log [H+]

=       - (log5 + log 10-4)

=       - [.699 - 4]

=       - .699+4

pH      =       3.301 (Answer)

Example. (3) Calculate the pH and pOH of a solution whose (H+) ion Concentration is 3.0xl0-2 moles/ litre?

Solution:       

pH      =       - log [H+]

=       - log (3xl0-2)

=       - (log 3-log 10-2)

=       - [0.477 - 2]

=       [-0.477 +2]

pH      =       1.523

As     pH      +       pOH    = 14

1.523  +       pOH    = 14

pOH    =       14 - 1.523

pOH    =       12.477

pOH    =       14 - 1.523

pOH    =       12.477

pH = 1.523   pOH = 12.477 (Answer)

Example. (4)    Calculate the (H+) ion concentration of a solution whose pH 4.4?

Solution:

 

pH      =       - log [H+]

pH      =       - log (H+) = 4.4

pH      =       log (H+) = -4.4

As we know that exponent should be whole number, so that -4.4 is equal to (-5+0.6), then we can write as:

log [H+] = - 4.4

log [H+] = (0.6-5)

H+ = Antilog (0.6 -5)

H+ = Antilog of 0.6 x Antilog of -5 = 3.98 x 105

H+ = 3.98 x 105 M   (Answer)

9.5.3   The Measurement of pH with (pH Paper):

There are three methods to measure the pH of a solution, (i) By acid-base indicator's. (ii) By"pH-meters" and (iii) By "pH-paper". The method for the approximate determination of pH widely used is with the 'pH-paper'. In this method, paper-strips that are treated with several different indicators can be used to estimate pH. These papers-strips are called "pH-paper". pH can be estimated by dipping the pH paper in a given solution, then by matching the colour appearing on the pH-paper with a colour corresponding to a known pH.

9.5.4 The Importance of pH:

The concept of pH plays essential role in the field of biology. Other areas in which pH information and control is necessary, include, water treatment, soil conditioning, swimming pool managements, corrosion, control, food processing and electroplating.

For example: The pH of human blood is normally maintained by the body between 7.35 and 7.45. If the blood pH drops to 7, as in some illness, the patient may go into coma, a pH below 6, death may occur. pH rises as high as 7.7 or 7.8 causes diabetes excess vomiting, diarrhea.

Hence the pH values of various body fluids are very important for a doctor in diagnosing and treating many illnesses.

The pH values of several biological fluids.

Fluid

pH

Lemon juice

2.3

Vinegar

2.8

Tomato juice

4.2

Human urine

5.0-7.0

Cow's milk

6.5

Saliva

7.0

Human blood

7.35-7.45

Egg white

7.8

 

 
    back to previous page  
     
 
 
Home | About Us | Notes | Animation | Career Guidance | Tuition | Wisdom | Contact Us
© All Copyright Reserved to www.mynoteslibrary.com