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Matters and Properties of Matters
Atomic Structure with Examples
Periodic Table
The Mole Concept with Examples
Gases with Examples
Chemical Reactions with Examples
Nuclear Chemistry (Radioactivity)
Acids and Bases
--Properties of Acids and Bases
--Strengths of Acids and Bases
--pH pOH and Ionization of Water
--Acids and Bases Cheat Sheet
Rates of Reactions (Chemical Kinetics)
Chemical Equilibrium
Chemical Bonds
Exams and Problem Solutions
Old Version


pH pOH and Ionization of Water

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 oC 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

nHCl=224/22400=0,01 mol

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.

Acids and Bases Exams and  Problem Solutions

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Tags: Acids and Bases Cheat Sheet |  Acids and Bases Exam1 and Problem Solutions | 

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