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IONISATION AND IONIC PRODUCT OF WATER (Kw)

Water is a polar ,protic inorganic compound that is at room temperature a tasteless and odourless liquid nearly colourless with hint of blue. It covers about 71.4% of earth. Water is universal solvent due to its ability to dissolved many substance .The IUPAC name of is Water  and  Oxidane.
Pure water is a very weak electrolyte -
On applying the law of mass action at equilibrium,
              Then
Since, ionization takes places to a very small extent, so the concentration of unionized water molecule is regarded as constant. Thus the product of K[H2O] gives another constant Kw.
                       So    
The product of concentration of H+ and OH. ion in water at a particular temperature is known as ionic  product of water.

IMPORTANT POINT ABOUT WATER:
(1) Mass of 1 litre of water = 997 gm.
(2) Molar concentration of water = 55.5 gm-mole / litre.
(3) Number of water molecule in 1 litre of water = 55.5×6.023×1023 = 3.34×1025.
(4) Concentration of H+ ion in one litre of neutral water = 10-7 moles / litre.
(5) Concentration of OH- ion in one litre of neutral water = 10-7 moles / litre.
(6) Number of H+ ion in one litre of neutral water = 6.023 x 1016.

(6) Number of OH.- ion in one litre of neutral water = 6.023 x 1016.  

IONIZATION CONSTANT (Ki) OR DESSOCIATION CONSTANT (Kd) OF WATER:
At 25Kw=1x10-14  and conc. of water 1000/18=55.55 mole /liter

Ki or Kd are also called acid or base constant of water.
Hence Pka and Pkb of water is equal (Pka=Pkb)
DISSOCIATION CONSTANT OF WATER (DOD):

Pure water is weak electrolyte and dissociates as

EFFECT OF TEMPERATURE ON IONIC PRODUCT OF WATER ( Kw):
(1) Ionisation constant of water is endothermic process, so on increasing temperature Keq increase. Kw increases with increase in temperature.
(2) During self ionisation of water dissociation concentration of water is remain constant=55.55 mole /liter.
(3) Kw is a thermodynamics equilibrium constant for water it depend upon only temperature, if temperature is increases then value of Kw is increases. and temperature is decreases then Kw decreases
(4) The numerical value of Kw increases considerably with temperature from 0.11 × 10–14 at 0°C to 50 × 10–14 at 100°C. It is 1.0 × 10–14 at 25°C which we will use frequently the variation of ionic product of water with temperature is given by
On the basis of above equation we calculate the value of kw at different temperature which are given below .

We know ionic product of water at 25 c is
The pH scale was marked from 0 to 14 with central point at 7 at 25ºC, taking water as solvent.
If the temperature and the solvent are changed, the pH range of the scale will also change. For example
 at 25 ºC (Kw = 10–14) Neutral point, pH= 7
 at 80 ºC (Kw = 10–13) Neutral point, pH = 6.5

ILLUSTRATIVE EXAMPLE (1): H2O has its pH 6.5 predict the nature of solution when
(a) Kw = 10–14 at 25 ºC
(b) Kw = 10–15 at 70 ºC
(c) Kw = 10–12 at 90 ºC
SOLUTION:
(a) acidic  (b) acidic (c) basic

ILLUSTRATIVE EXAMPLE (2): The ionization constant for water is 1x10-13.6 at 37ºC.What will be H3O+ and OH-Concentration at that temperature.
(1) 3.75 x10-8          (2) 1.75 x 10-8                      (3) 1.58 x 10-7            (4) 1.85 x 10-8

SOLUTION:
ILLUSTRATIVE EXAMPLE (3): Calculate the ionic product of water at 25c if Kw is 1.0 X10-14 m2 at 25c
(Ans - 5.5x10-14)

ILLUSTRATIVE EXAMPLE (4): At 25 c the degree of dissociation of water is 1.8 x 10-9 calculate the dissociation constant and ionic product of water.
(Ans-3.24x10-18)
ILLUSTRATIVE EXAMPLE (5): At 30 c the self ionization constant of liquid ammonia (NH3) is 10-30.the density of NH3 at -30 c is 0.85 gm /liter. 
(1) Calculate the ionic product of liquid ammonia?
(2) Calculate % extent of self ionization of NH3 molecules?
(3) How many NH4+ ions are present in 5ml of liquid NH3?
(1- 2.5x10-27 m2, 2- 10-13% 3- 25x10-6x6.022x1023 )
ILLUSTRATIVE EXAMPLE (6):  At 25c the ionic product of heavy water (D2O) is 1.44x 10-15 m2 Calculate it's dissociation constant and degree of dissociation (density of D2O=1.02) gm/ml)
(Ans-7.44x10-10)                                               
                                        ***

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