Chemistry · Class 12 Science
Ch 3Ionic Equilibria — Class 12 Chemistry, concept-first.
The equilibrium between ions and unionized molecules in solution is called ionic equilibrium. The principles of chemical equilibrium we studied in Standard XI will be applied to ionic equilibria.
Key concepts
Hover a concept to preview it and jump to its most relevant Q&A.
Ionization Constant (Ka, Kb)
For a weak acid HA that dissociates partially, , the acid-dissociation constant is ; for a weak base BOH, , the base-dissociation constant is . A larger Ka or Kb means a stronger (more dissociated) weak acid or base.
Most relevant Q&A
- A weak monobasic acid is 0.05% dissociated in 0.02 M solution. Calculate dissociation constant of the acid.Free
- The dissociation constant of $\mathrm{NH_4OH}$ is $1.8 \times 10^{-5}$. Calculate its degree of dissociation in 0.01 M solution.Free
- Calculate $\mathrm{[H_3O^+]}$ in 0.1 mol $\mathrm{dm^3}$ solution of acetic acid. Given : $K_a\ \mathrm{[CH_3COOH]} = 1.8 \times 10^{-5}$ >…Preview
- **i.** Define degree of dissociation. Derive Ostwald's dilution law for the $\mathrm{CH_3COOH}$.Free
- **iv.** Dissociation constant of acetic acid is $1.8 \times 10^{-5}$. Calculate percent dissociation of acetic acid in 0.01 M solution.Preview
In previous exams
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Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Introduction
The equilibrium between ions and unionized molecules in solution is called ionic equilibrium. The principles of chemical equilibrium we studied in Standard XI will be applied to ionic equilibria.
+−Can you recall?i2 questions
Types of electrolyte
The substances which give rise to ions when dissolved in water are electrolytes. The non electrolytes are those which do not ionize and exist as molecules in aqueous solutions.
Strong electrolyte
The electrolytes ionizing completely or almost completely are strong electrolytes. For example : strong acids, strong bases and salts.
Weak electrolyte
The electrolytes which dissociate to a smaller extent in aqueous solution are weak electrolytes. Weak acids and weak bases belong to this class.
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Degree of dissociation (α)
The degree of dissociation of an electrolyte is defined as a fraction of the total number of moles of the electrolyte that dissociates into its ions when the equilibrium is attained.
Acids and Bases
Acids and bases are familiar chemical compounds. Acetic acid is found in vinegar, citric acid in lemons, magnesium hydroxide in antacids, ammonia in household cleaning products.
Arrhenius theory of acids and bases
According to this theory acids and bases are defined as follows :
Bronsted - Lowry theory
J. N. Bronsted and T. M. Lowry (1923) proposed a more general theory known as the Bronsted-Lowry proton transfer theory. According to this theory acids and bases are defined as follows.
Lewis theory
A more generalized acid-base concept was put forward by G. N. Lewis in 1923. According to this theory acids and bases are defined as follows.
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Ionisation of acids and bases
Acids and bases are classified as strong acids and strong bases, weak acids and weak bases on the basis of their extent of dissociation.
Dissociation constant of weak acids and weak bases
The dissociation of a weak acid HA in water is expressed as
Ostwald's dilution law
Arrhenius concept of acids and bases was expressed quantitatively by F. W. Ostwald in the form of the dilution law in 1888.
+−Problemsi4 questions
- Problem 3.1A weak monobasic acid is 0.05% dissociated in 0.02 M solution. Calculate dissociation constant of the acid.Free
- Problem 3.2The dissociation constant of $\mathrm{NH_4OH}$ is $1.8 \times 10^{-5}$. Calculate its degree of dissociation in 0.01 M solution.Free
- Problem 3.3A weak monobasic acid is 12% dissociated in 0.05 M solution. What is percent dissociation in 0.15 M solution.Preview
- Problem 3.4Calculate $\mathrm{[H_3O^+]}$ in 0.1 mol $\mathrm{dm^3}$ solution of acetic acid. Given : $K_a\ \mathrm{[CH_3COOH]} = 1.8 \times 10^{-5}$ >…Preview
Autoionization of water
Pure water ionizes to a very small extent. The ionization equilibrium of water is represented as,
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pH Scale
Instead of writing concentration of ions in mol dm, sometimes it is convenient to express it on the logarithmic scale. This is known as pH scale.
Relationship between pH and pOH
The ionic product of water is
Acidity, basicity and neutrality of aqueous solutions
3 Q1. Neutral solution : For pure water or any aqueous neutral solution at 298 K
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Hydrolysis of salts
A salt is produced when an acid reacts with a base. Whether the aqueous solution of a salt is neutral, acidic or basic depends on how its ions interact with water -- the subject of the subsections tha…
Types of salts
These are of four types
Concept of hydrolysis
When a salt is dissociated in water, it dissociates completely into its constituent ions. The solvent water dissociates slightly as,
Salts of strong acids and strong bases
NaCl is a salt of strong acid HCl and a strong base NaOH. When it is dissolved in water, it dissociates completely into its ions.
Salts of strong acids and weak bases
is salt of strong acid and weak base . When is dissolved in water, it dissociates completely as,
Salts of weak acids and strong bases
is a salt of weak acid and strong base NaOH. When dissolved in water, it dissociates completely.
+−Can you tell? 3.7.51 question
Salts of weak acids and weak bases
When salt BA of weak acid HA and weak base BOH is dissolved in water, it dissociates completely as
Buffer solutions
Buffer solution is defined as a solution which resists drastic changes in pH when a small amount of strong acid or strong base or water is added to it.
Types of buffer solutions
There are two types of buffer solutions. Acidic buffer is used to maintain an acidic pH, while basic buffer maintains alkaline pH.
Buffer action
Let us consider sodium acetate - acetic acid buffer. Here sodium acetate is a strong electrolyte which dissociates completely in water producing large concentration of as follows :
Properties of buffer solution
The pH of a buffer solution does not change appreciably
+−Can you tell? 3.8.31 question
Applications of buffer solution
Buffer solution finds extensive applications in a variety of fields. Some of its applications are given.
+−Problemsi2 questions
- Problem 3.9Calculate the pH of buffer solution containing 0.05 mol NaF per litre and 0.015 mol HF per litre. [$K_a = 7.2 \times 10^{-4}$ for HF]Free
- Problem 3.10Calculate the pH of buffer solution composed of 0.1 M weak base BOH and 0.2 M of its salt BA. [$K_b = 1.8\times 10^{-5}$ for the weak base]Preview
Solubility product
The dissolution and precipitation of a sparingly soluble ionic compound is governed by an equilibrium constant of its own -- the solubility product -- developed in the subsections below.
Solubility equilibria
Hereafter we confine our attention to sparingly soluble compounds, that is, compounds those dissolve only slightly in water.
Relationship between solubility and solubility product
The solubility of a compound is the amount in grams that dissolves per unit volume (which may be 100 mL or 1 L) of its saturated solution.
Condition of precipitation
Ionic product () of an electrolyte is defined in the same way as solubility product (). The only difference is that the ionic product expression contains concentration of ions under any condition wher…
+−Problemsi3 questions
- Problem 3.11A solution is prepared by mixing equal volumes of 0.1M $\mathrm{MgCl_2}$ and 0.3M $\mathrm{Na_2C_2O_4}$ at 293 K. Would $\mathrm{MgC_2O_4}$…Free
- Problem 3.12The solubility product of AgBr is $5.2 \times 10^{-13}$. Calculate its solubility in mol $\mathrm{dm^{-3}}$ and g $\mathrm{dm^{-3}}$(Molar m…Preview
- Problem 3.13If 20.0 $\mathrm{cm^3}$ of 0.050 M $\mathrm{Ba(NO_3)_2}$ are mixed with 20.0 $\mathrm{cm^3}$ of 0.020 M NaF, will $\mathrm{BaF_2}$ precipita…Preview
Common ion effect
2 QConsider a solution of weak acid and its soluble ionic salt .
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Common ion effect and solubility
The presence of a common ion also affects the solubility of a sparingly soluble salt. Consider, the solubility equilibrium of AgCl,
1. Choose the most correct answer :
+−Choose the most correct answer7 questions
- Q1**i.** The pH of $10^{-8}$ M of HCl is a. 8 b. 7 c. less than 7 d. greater than 7Free
- Q2**ii.** Which of the following solution will have pH value equal to 1.0 ? a. 50 mL of 0.1M HCl + 50mL of 0.1M NaOH b. 60 mL of 0.1M HCl + 40…Free
- Q3**iii.** Which of the following is a buffer solution ? a. $\mathrm{CH_3COONa}$ + NaCl in water b. $\mathrm{CH_3COOH}$ + HCl in water c. $\ma…Free
- Q4**iv.** The solubility product of a sparingly soluble salt AX is $5.2 \times 10^{-13}$. Its solubility in $\mathrm{mol\ dm^{-3}}$ is a. $7.2…Preview
- Q5**v.** Blood in human body is highly buffered at pH of a. 7.4 b. 7.0 c. 6.9 d. 8.1Preview
- Q6**vi.** The conjugate base of $\mathrm{[Zn(H_2O)_4]^{2+}}$ is a. $\mathrm{[Zn(H_2O)_4]^{2-}NH_3}$ b. $\mathrm{[Zn(H_2O)_3]^{2-}}$ c. $\mathr…Preview
- Q7**vii.** For pH > 7 the hydronium ion concentration would be a. $10^{-7}$ M b. $< 10^{-7}$ M c. $> 10^{-7}$ M d. $\geq 10^{-7}$ MPreview
2. Answer the following in one sentence :
+−Answer the following in one sentence10 questions
- Q1**i.** Why cations are Lewis acids ?Free
- Q2**ii.** Why is KCl solution neutral to litmus?Free
- Q3**iii.** How are basic buffer solutions prepared?Free
- Q4**iv.** Dissociation constant of acetic acid is $1.8 \times 10^{-5}$. Calculate percent dissociation of acetic acid in 0.01 M solution.Preview
- Q5**v.** Write one property of a buffer solution.Preview
- Q6**vi.** The pH of a solution is 6.06. Calculate its $\mathrm{H^+}$ ion concentration.Preview
- Q7**vii.** Calculate the pH of 0.01 M sulphuric acid.Preview
- Q8**viii.** The dissociation of $\mathrm{H_2S}$ is suppressed in the presence of HCl. Name the phenomenon.Preview
- Q9**ix.** Why is it necessary to add $\mathrm{H_2SO_4}$ while preparing the solution of $\mathrm{CuSO_4}$?Preview
- Q10**x.** Classify the following buffers into different types : a. $\mathrm{CH_3COOH}$ + $\mathrm{CH_3COONa}$ b. $\mathrm{NH_4OH}$ + $\mathrm{N…Preview
3. Answer the following in brief :
+−Answer the following in brief10 questions
- Q1**i.** What are acids and bases according to Arrhenius theory ?Free
- Q2**ii.** What is meant by conjugate acid-base pair?Free
- Q3**iii.** Label the conjugate acid-base pair in the following reactions a. $\mathrm{HCl + H_2O \rightleftharpoons H_3O^+ + Cl^-}$ b. $\mathrm…Free
- Q4**iv.** Write a reaction in which water acts as a base.Preview
- Q5**v.** Ammonia serves as a Lewis base whereas $\mathrm{AlCl_3}$ is Lewis acid. Explain.Preview
- Q6**vi.** Acetic acid is 5% ionised in its decimolar solution. Calculate the dissociation constant of acid (Ans : $2.63 \times 10^{-4}$)Preview
- Q7**vii.** Derive the relation pH + pOH = 14.Preview
- Q8**viii.** Aqueous solution of sodium carbonate is alkaline whereas aqueous solution of ammonium chloride is acidic. Explain.Preview
- Q9**ix.** pH of a weak monobasic acid is 3.2 in its 0.02 M solution. Calculate its dissociation constant.Preview
- Q10**x.** In NaOH solution $\mathrm{[OH^-]}$ is $2.87 \times 10^{-4}$. Calculate the pH of solution.Preview
4. Answer the following :
+−Answer the following9 questions
- Q1**i.** Define degree of dissociation. Derive Ostwald's dilution law for the $\mathrm{CH_3COOH}$.Free
- Q2**ii.** Define pH and pOH. Derive relationship between pH and pOH.Free
- Q3**iii.** What is meant by hydrolysis ? A solution of $\mathrm{CH_3COONH_4}$ is neutral. why ?Free
- Q4**iv.** Dissociation of HCN is suppressed by the addition of HCl. Explain. > [!NOTE] > In the textbook the printed numbering of this exercis…Preview
- Q5**vi.** Derive the relationship between degree of dissociation and dissociation constant in weak electrolytes. > [!NOTE] > The textbook numb…Preview
- Q6**vii.** Sulfides of cation of group II are precipitated in acidic solution ($\mathrm{H_2S}$ + HCl) whereas sulfides of cations of group III…Preview
- Q7**viii.** Solubility of a sparingly soluble salt get affected in presence of a soluble salt having one common ion. Explain.Preview
- Q8**ix.** The pH of rain water collected in a certain region of Maharashtra on particular day was 5.1. Calculate the $\mathrm{H^+}$ ion concen…Preview
- Q9**x.** Explain the relation between ionic product and solubility product to predict whether a precipitate will form when two solutions are m…Preview
Activity
Sample & Board Papers
Sample papers and previous-year board questions for this subject.
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- Q1The value of $[H_3O^+]$ in mol $lit^{-1}$ of 0.001 M acetic acid solution ($K_a = 1.8 \times 10^{-5}$) is _____. (a) $1.34 \times 10^{-1}$ (…Preview
- Q2Calculate the pH of buffer solution composed of 0.01 M weak base BOH and 0.02 M of its salt BA. [$K_b = 1.8 \times 10^{-5}$ for weak base]Preview
- Q3The pH of weak monoacidic base is 11.2, its $OH^-$ ion concentration is: (a) $1.585 \times 10^{-3}\ mol\ dm^{-3}$ (b) $3.010 \times 10^{-11}…Preview
- Q4Define: Acidic buffer solution. Write the relationship between solubility and solubility product for $PbI_2$.Preview
- Q5A weak monobasic acid is 10% dissociated in 0.05 M solution. What is percent dissociation in 0.15 M solution?Preview
- Q6The pH of 0.001M HCl solution is _____. (a) 10 (b) 3 (c) 2 (d) 11Preview
- Q7Explain buffer action of sodium acetate-acetic acid buffer.Preview
- Q8Define acids and bases according to Bronsted-Lowry theory. Derive relationship between pH and pOH.Preview
- Q9Which formula co-relates degree of dissociation and concentration of electrolyte? (a) $c=\sqrt{\dfrac{K_a}{\alpha}}$ (b) $\alpha=\sqrt{\dfra…Preview
- Q10Define conjugate acid-base pair. The hydroxyl ion concentration in aqueous solution of NaOH is 2 × 10$^{-4}$ mol dm$^{-3}$. Calculate pH of…Preview
- Q11The pOH of 0.01 M HCl solution is _____. (a) 1 (b) 2 (c) 11 (d) 12Preview
- Q12(i) Define: Common ion effect. (ii) Write preparation of glucose from starch.Preview
- Q13(i) Acetic acid is 5% ionised in its decimolar solution. Calculate the dissociation constant of acetic acid. (ii) Write chemical formula of…Preview