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Chemistry · Class 12 Science

Ch 8Ionic Equilibrium — Class 12 Chemistry, concept-first.

Class XI introduced chemical equilibrium in general. This unit narrows the focus to ionic equilibria -- specifically the equilibria acids and bases set up in solution -- and it isn't just textbook chemistry: several vital processes inside your own body run on aqueous equilibria, such as the carbonic acid-bicarbonate bu…

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Key concepts

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Solubility Product and Common Ion Effect

For a sparingly soluble salt , the solubility product is -- the product of the constituent ions' molar concentrations, each raised to its stoichiometric coefficient, once the solid's own (constant) concentration is absor…

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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

Class XI introduced chemical equilibrium in general. This unit narrows the focus to ionic equilibria -- specifically the equilibria acids and bases set up in solution -- and it isn't just textbook che…

8.1

Acids and Bases

The word 'acid' comes from the Latin acidus, meaning sour, and describes substances long recognised by their sour taste, their reaction with active metals such as zinc to liberate hydrogen gas, and th…

8.1.1

Arrhenius Concept

Svante Arrhenius's theory (one of the earliest systematic acid-base concepts) defines an acid as a substance that dissociates in water to give hydrogen ions -- for example and -- and a base as a subst…

8.1.2

Lowry – Bronsted Theory (Proton Theory)

In 1923, Lowry and Bronsted proposed a more general definition: an acid is a substance with a tendency to donate a proton to another substance, and a base is a substance with a tendency to accept a pr…

8.1.3

Lewis Concept

In 1923, Gilbert N. Lewis proposed the most general acid-base concept of all, built on the electron pair rather than the proton: a Lewis acid is a species that accepts an electron pair, and a Lewis ba…

8.2

Strength of Acids and Bases

The strength of an acid or base is set by how much or it produces per mole dissolved in water: a strong acid/base is almost completely dissociated, while a weak acid/base is only partially dissociated…

8.3

Ionisation of Water

Pure water itself has a slight tendency to dissociate -- one water molecule donates a proton to another, called the auto-ionisation of water: , in which one molecule acts as the acid and the other as…

8.4

The pH Scale

Acid/base solutions are usually dealt with in the concentration range to M -- an inconveniently wide span to express directly.

8.4.1

Relation between pH and pOH

Adding the defining equations and gives . At , , so , giving the widely used relation at . This lets pH and pOH be converted into each other directly once one of them (or /) is known.

8.5

Ionisation of Weak Acids

Weak acids are only partially dissociated in water, leaving an equilibrium between the undissociated acid and its ions. For the ionisation of a weak monobasic acid , the equilibrium constant is .

8.5.1

Ostwald's Dilution Law

Ostwald's dilution law relates a weak acid's dissociation constant to its degree of dissociation (the fraction of the total moles that dissociates at equilibrium, ) and its concentration .

8.6

Common Ion Effect

When a salt of a weak acid is added to that same weak acid, the acid's dissociation is suppressed further -- called the common ion effect.

8.7

Buffer Solution

Our blood maintains a nearly constant pH despite countless cellular acid-base reactions, because of buffer action.

8.7.1

Buffer Action

To resist pH changes on adding an acid or base, a buffer must contain both an acidic and a basic component that can neutralise the added stress without consuming each other.

8.7.2

Buffer Capacity and Buffer Index

The buffering ability of a solution is quantified by its buffer capacity. Van Slyke introduced the buffer index, , where is the number of gram-equivalents of acid or base added to one litre of buffer…

8.7.3

Henderson – Hasselbalch Equation

In an acidic buffer, ; since a weak acid dissociates only slightly, and common-ion suppression from the salt weakens it further, the equilibrium acid concentration is close to the acid's initial (unio…

8.8

Salt Hydrolysis

When an acid reacts with a base, a salt and water are formed -- neutralisation. Salts dissociate completely in water into their constituent ions, which are themselves hydrated; in some cases the catio…

8.8.1

Salts of Strong Acid and Strong Base

Consider . The salt dissociates completely, . is the conjugate base of the strong acid and so has no real tendency to react with ; is the conjugate acid of the strong base and has no real tendency to…

8.8.2

Hydrolysis of Salt of Strong Base and Weak Acid (Anionic Hydrolysis)

Consider . dissociates completely, . is the conjugate base of the weak acid , so it does have a real tendency to pull a proton from water, , while has no such tendency toward ; the result is , so the…

8.8.3

Hydrolysis of Salt of Strong Acid and Weak Base (Cationic Hydrolysis)

Consider , which dissociates completely, . is the conjugate acid of the weak base , so it has a real tendency to donate a proton to water, , while shows no such tendency toward ; the result is , so th…

8.8.4

Hydrolysis of Salt of Weak Acid and Weak Base

Consider ammonium acetate, , where BOTH ions hydrolyse: and . Whether the resulting solution is acidic, basic or neutral now depends purely on which parent was the weaker: if , the solution is acidic…

8.9

Solubility Product

Precipitation reactions in qualitative inorganic analysis -- such as dilute precipitating as sparingly soluble , or calcium oxalate precipitating in kidney stones -- are understood through the solubil…

8.9.1

Determination of Solubility Product from Molar Solubility

can be calculated from the molar solubility -- the maximum number of moles of solute dissolving per litre.

SUMMARY

A consolidated recap of the unit: the Arrhenius, Lowry-Bronsted and Lewis definitions of acids and bases (proton donor/acceptor, then electron-pair acceptor/donor); the ionic product of water ; the pH…

EVALUATION

50 Q

The chapter-end evaluation. Part I ('Choose the correct answer') is 23 multiple-choice questions spanning Bronsted-Lowry and Lewis acid-base theory, conjugate pairs, pH/pOH calculations for strong-aci…

+Choose the Best Answer23 questions
  1. Q1Concentration of the $Ag^+$ ions in a saturated solution of $Ag_2C_2O_4$ is $2.24 \times 10^{-4}$ mol L$^{-1}$. Solubility product of $Ag_2C…Free
  2. Q2Following solutions were prepared by mixing different volumes of NaOH and HCl of different concentrations. (NEET – 2018) i. 60 mL $\frac{M}{…Free
  3. Q3The solubility of $BaSO_4$ in water is $2.42 \times 10^{-3}$ g L$^{-1}$ at 298 K. The value of its solubility product ($K_{sp}$) will be (NE…Free
  4. Q4pH of a saturated solution of $Ca(OH)_2$ is 9. The solubility product ($K_{sp}$) of $Ca(OH)_2$ is a) $0.5 \times 10^{-15}$ b) $0.25 \times 1…Preview
  5. Q5Conjugate base for Bronsted acids $H_2O$ and $HF$ are a) $OH^-$ and $H_2F^+$, respectively b) $H_3O^+$ and $F^-$, respectively c) $OH^-$ and…Preview
  6. Q6Which will make a basic buffer? a) 50 mL of 0.1M NaOH + 25 mL of 0.1M $CH_3COOH$ b) 100 mL of 0.1M $CH_3COOH$ + 100 mL of 0.1M $NH_4OH$ c) 1…Preview
  7. Q7Which of the following fluoro compounds is most likely to behave as a Lewis base? (NEET – 2016) a) $BF_3$ b) $PF_3$ c) $CF_4$ d) $SiF_4$Preview
  8. Q8Which of these is not likely to act as a Lewis base? a) $BF_3$ b) $PF_3$ c) CO d) $F^-$Preview
  9. Q9The aqueous solutions of sodium formate, anilinium chloride and potassium cyanide are respectively a) acidic, acidic, basic b) basic, acidic…Preview
  10. Q10The percentage of pyridine ($C_5H_5N$) that forms pyridinium ion ($C_5H_5NH^+$) in a 0.10M aqueous pyridine solution ($K_b$ for $C_5H_5N = 1…Preview
  11. Q11Equal volumes of three acid solutions of pH 1, 2 and 3 are mixed in a vessel. What will be the $H^+$ ion concentration in the mixture? a) $3…Preview
  12. Q12The solubility of AgCl(s) with solubility product $1.6 \times 10^{-10}$ in 0.1M NaCl solution would be a) $1.26 \times 10^{-5}$ M b) $1.6 \t…Preview
  13. Q13If the solubility product of lead iodide is $3.2 \times 10^{-8}$, its solubility will be a) $2 \times 10^{-3}$ M b) $4 \times 10^{-4}$ M c)…Preview
  14. Q14MY and $NY_3$ are insoluble salts and have the same $K_{sp}$ values of $6.2 \times 10^{-13}$ at room temperature. Which statement would be t…Preview
  15. Q15What is the pH of the resulting solution when equal volumes of 0.1M NaOH and 0.01M HCl are mixed? a) 2.0 b) 3 c) 7.0 d) 12.65Preview
  16. Q16The dissociation constant of a weak acid is $1 \times 10^{-3}$. In order to prepare a buffer solution with a pH = 4, the $\frac{[Acid]}{[Sal…Preview
  17. Q17The pH of $10^{-5}$ M KOH solution will be a) 9 b) 5 c) 19 d) none of thesePreview
  18. Q18$H_2PO_4^-$ is the conjugate base of a) $PO_4^{3-}$ b) $P_2O_5$ c) $H_3PO_4$ d) $HPO_4^{2-}$Preview
  19. Q19Which of the following can act as a Lowry–Bronsted acid as well as a base? a) HCl b) $SO_4^{2-}$ c) $HPO_4^{2-}$ d) $Br^-$Preview
  20. Q20The pH of an aqueous solution is zero. The solution is a) slightly acidic b) strongly acidic c) neutral d) basicPreview
  21. Q21The hydrogen ion concentration of a buffer solution consisting of a weak acid and its salt is given by a) $[H^+] = K_a \dfrac{[acid]}{[salt]…Preview
  22. Q22Which of the following relations is correct for the degree of hydrolysis of ammonium acetate? a) $h = \sqrt{\dfrac{K_h}{C}}$ b) $h = \sqrt{\…Preview
  23. Q23Dissociation constant of $NH_4OH$ is $1.8 \times 10^{-5}$. The hydrolysis constant of $NH_4Cl$ would be a) $1.8 \times 10^{-19}$ b) $5.55 \t…Preview
+Write Brief Answer27 questions
  1. Q1What are Lewis acids and bases? Give two examples for each.Free
  2. Q2Discuss the Lowry – Bronsted concept of acids and bases.Free
  3. Q3Identify the conjugate acid–base pair for the following reactions in aqueous solution: i) $HS^-(aq) + HF \rightleftharpoons F^-(aq) + H_2S(a…Free
  4. Q4Account for the acidic nature of $HClO_4$ in terms of Bronsted – Lowry theory, and identify its conjugate base.Preview
  5. Q5When aqueous ammonia is added to $CuSO_4$ solution, the solution turns deep blue due to the formation of the tetraamminecopper(II) complex,…Preview
  6. Q6The concentration of hydroxide ion in a water sample is found to be $2.5 \times 10^{-6}$ M. Identify the nature of the solution.Preview
  7. Q7A lab assistant prepared a solution by adding a calculated quantity of HCl gas at $25^\circ C$ to get a solution with $[H_3O^+] = 4 \times 1…Preview
  8. Q8Calculate the pH of 0.04 M $HNO_3$ solution.Preview
  9. Q9Define solubility product.Preview
  10. Q10Define ionic product of water. Give its value at room temperature.Preview
  11. Q11Explain common ion effect with an example.Preview
  12. Q12Derive an expression for Ostwald's dilution law.Preview
  13. Q13Define pH.Preview
  14. Q14Calculate the pH of $1.5 \times 10^{-3}$ M solution of $Ba(OH)_2$.Preview
  15. Q1550 mL of 0.05M $HNO_3$ is added to 50 mL of 0.025M KOH. Calculate the pH of the resultant solution.Preview
  16. Q16The $K_a$ value for HCN is $10^{-9}$. What is the pH of 0.4M HCN solution?Preview
  17. Q17Calculate the extent of hydrolysis and the pH of 0.1M ammonium acetate. Given that $K_a = K_b = 1.8 \times 10^{-5}$.Preview
  18. Q18Derive an expression for the hydrolysis constant and degree of hydrolysis of the salt of a strong acid and a weak base.Preview
  19. Q19Solubility product of $Ag_2CrO_4$ is $1 \times 10^{-12}$. What is the solubility of $Ag_2CrO_4$ in 0.01M $AgNO_3$ solution?Preview
  20. Q20Write the expression for the solubility product of $Ca_3(PO_4)_2$.Preview
  21. Q21A saturated solution, prepared by dissolving $CaF_2(s)$ in water, has $[Ca^{2+}] = 3.3 \times 10^{-4}$ M. What is the $K_{sp}$ of $CaF_2$?Preview
  22. Q22$K_{sp}$ of AgCl is $1.8 \times 10^{-10}$. Calculate its molar solubility in 1M $AgNO_3$.Preview
  23. Q23A particular saturated solution of silver chromate $Ag_2CrO_4$ has $[Ag^+] = 5 \times 10^{-5}$ M and $[CrO_4^{2-}] = 4.4 \times 10^{-4}$ M.…Preview
  24. Q24Write the expression for the solubility product of $Hg_2Cl_2$.Preview
  25. Q25$K_{sp}$ of $Ag_2CrO_4$ is $1.1 \times 10^{-12}$. What is the solubility of $Ag_2CrO_4$ in 0.1M $K_2CrO_4$?Preview
  26. Q26Will a precipitate be formed when 0.150 L of 0.1M $Pb(NO_3)_2$ and 0.100 L of 0.2M NaCl are mixed? $K_{sp}(PbCl_2) = 1.2 \times 10^{-5}$.Preview
  27. Q27$K_{sp}$ of $Al(OH)_3$ is $1 \times 10^{-15}$. At what pH does $1.0 \times 10^{-3}$ M $Al^{3+}$ precipitate on the addition of a buffer of $…Preview

ICT Corner

A hands-on activity box titled 'Buffers and pH', directing students via a URL/QR code to an online virtual-lab simulator.

Sample & Board Papers

Sample papers and previous-year board questions for this subject.

+Show 20 questions20 questions
  1. Q1For the titration between hydrochloric acid and sodium carbonate, the indicator used is : (a) potassium permanganate (b) phenolphthalein (c)…Preview
  2. Q2Write three significances of Henderson equation.Preview
  3. Q3(a) Explain Ostwald's dilution law. (b) Mention the IUPAC conventions for writing cell diagram.Preview
  4. Q4$NH_4OH$ is a weak base because : (a) it is completely ionised (b) it has low vapour pressure (c) it has low density (d) it is only partiall…Preview
  5. Q5Define reaction quotient.Preview
  6. Q6What is the indicator used for the titration between oxalic acid and sodium hydroxide ? Justify your answer with suitable reason.Preview
  7. Q7State of chemical equilibrium is : (a) irreversible (b) stationary and irreversible (c) dynamic (d) stationaryPreview
  8. Q8What is Reaction Quotient ?Preview
  9. Q9(a) (i) Mention the dispersed phase and dispersion medium present in the following colloids. (A) Paint (B) Froths of air (ii) Write short no…Preview
  10. Q10Which of the following can act as Lowry-Bronsted acid as well as base ? (a) $HPO_4^{2-}$ (b) $HCl$ (c) $Br^-$ (d) $SO_4^{2-}$Preview
  11. Q11What are the limitations of Arrhenius concept ?Preview
  12. Q12Which one of the following will cause common-ion-effect when added to the following dissociation equilibrium reaction ? $CH_3COOH_{(aq)} \ri…Preview
  13. Q13What is conjugate Acid-Base pairs ?Preview
  14. Q14(a) Derive an expression for Ostwald's Dilution Law. **OR** (b) (i) Why aniline does not undergo Friedel Craft's reaction ? (ii) How nylon-2…Preview
  15. Q15Among the following which will not be hydrolysed ? (a) Sodium Chloride (b) Sodium Formate (c) Ammonium Formate (d) Ammonium NitratePreview
  16. Q16Explain common ion effect with an example.Preview
  17. Q17At $25\degree C$, ionic product constant $K_w$ of water is $1.00 \times 10^{-14}$. Its value at $40\degree C$ is ________. (a) $1.00 \times…Preview
  18. Q18Define Solubility Product.Preview
  19. Q19The pH of sodium formate solution in terms of $K_a$ and the concentration of the electrolyte is : (a) $pH = 7 - \dfrac{1}{2}pK_a - \dfrac{1}…Preview
  20. Q20Calculate the pH of $1.5 \times 10^{-3}$ M solution of $Ba(OH)_2$.Preview