Ionization of Water
Water ionize as given below;
H2O(l) ↔ H+(aq) + OH-(aq)
In pure water concentrations of H+ and OH- ions are equal to each other and at 25 0C, they have concentration 1x10-7 M. Since concentration of ion in pure water is too low, it is a bad electric conductor.
As in the case of pure water mediums having [H+]=[OH-] concentration are called neutral mediums. In water solutions multiplication of [H+] and [OH-] is constant and at 25 0C it is 1x10-14. This number is also called ionization constant of pure water.
If acid is added to pure water;
[H+]>1x10-7 M and [OH-]<1x10-7
If base is added to pure water;
[OH-]>1x10-7 M and [H+]<1x10-7
To sum up we can say that;
- If concentration of [H+] = [OH-]=10-7M, then solution is neutral.
- If concentration of [H+] > [OH-] or [H+]>10-7M and [OH-]<10-7 M, then solution is acidic.
- If concentration of [OH-]>[H+] or [H+]<10-7M and [OH-]>10-7 M, then solution is basic.
Example: HCl having volume 224 cm3 under standard conditions mixed with pure water and form 1 L solution. Which ones of the following statements are true for this solution?
I. Concentration of solution is 10-2 molar.
II. Concentration of H+ ion is 10-2 molar.
III. Concentration of OH- ion is 10-12 molar.
We find mole of HCl gas under standard conditions.
1 mole gas has volume 22,4 L and 1 L= 1000 cm3
Molarity of HC solution is;
[HCl]=0,01/1=0,01 M or [HCl]=10-2 M I is true
Since HCl is strong acid, it is completely ionize in solution. Thus, concentration of H+ ion is equal to concentration of HCl. II is true.
[OH-]=10-12 molar III is true.
pH and pOH
In liquid solutions, to state concentrations of H+ and OH- ions pH and pOH concepts are used. We can show pH and pOH in terms of concentration as;
Solution having molar concentration of H+=10-2 M has pH=2 and solution having molar concentration of OH-=10-5 has pOH=5. To remember logarithmic calculations;
[H+].[OH-]=10-14 log of this equation;
log[H+] + log[OH-]=-14
-log[H+] - log[OH-]=14
pH + pOH = 14
In acid solutions, [H+]>10-7 or pH<7
In base solutions [H+]<10-7 or pH>7
- If 7>pH>0 acidic solution
- If 14>pH>7 basic solution
- If pH=7 neutral solution
Picture given below summarizes what we try to explain above;
Example: Which ones of thee following statements are true for water solutions?
I. If pH=pOH=7 , then solution is neutral
II. If [H+]>10-7 then pH<7
III. If [OH-]>[H+] then pH<7
I is true is pH=pOH=7
If [H+]>10-7 M then pH<7 II is true
If [OH-]>[H+] then pH>7 III is false.
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