Solubility expressions MCQs With Answer

Solubility expressions MCQs With Answer are essential for B. Pharm students studying solubility equilibria, Ksp, molar solubility and factors affecting dissolution of ionic drugs. This concise, keyword-rich introduction covers solubility product constant (Ksp), common ion effect, complexation, pH dependence, ionic strength and temperature influences on saturated solutions. Understanding how to derive Ksp expressions, calculate molar solubility and predict precipitation is crucial for formulation, drug stability and analytical separations. These MCQs focus on practical calculations and conceptual depth to strengthen problem-solving skills in pharmaceutics and physical chemistry. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What does the solubility product constant (Ksp) represent for a slightly soluble ionic salt?

• The equilibrium constant for the dissolution of a solid salt into its constituent ions
• The rate constant for dissolution
• The concentration of undissolved solid in a saturated solution
• The ionic strength of the solution

Correct Answer: The equilibrium constant for the dissolution of a solid salt into its constituent ions

Q2. For a salt AB that dissolves as AB ⇌ A+ + B-, what is the correct Ksp expression?

• Ksp = [A+]
• Ksp = [B-]
• Ksp = [A+][B-]
• Ksp = [A+] + [B-]

Correct Answer: Ksp = [A+][B-]

Q3. For a salt A2B that dissolves as A2B ⇌ 2A+ + B2-, which Ksp expression is correct?

• Ksp = [A+]2[B2-]
• Ksp = [A+]^2[B2-]
• Ksp = [A+][B2-]^2
• Ksp = [A+]^3[B2-]

Correct Answer: Ksp = [A+]^2[B2-]

Q4. What is the Ksp expression for CaF2 dissolving as CaF2 ⇌ Ca2+ + 2F-?

• Ksp = [Ca2+][F-]
• Ksp = [Ca2+][F-]^2
• Ksp = [Ca2+]^2[F-]
• Ksp = [Ca2+]^2[F-]^2

Correct Answer: Ksp = [Ca2+][F-]^2

Q5. If the molar solubility of AgCl in pure water is s, which equation relates s to Ksp (AgCl)?

• Ksp = s
• Ksp = s^2
• Ksp = 2s^2
• Ksp = 4s^3

Correct Answer: Ksp = s^2

Q6. For CaF2 dissolving with molar solubility s, which expression gives Ksp in terms of s?

• Ksp = s^2
• Ksp = 4s^3
• Ksp = s^3
• Ksp = 2s^2

Correct Answer: Ksp = 4s^3

Q7. What is the Ksp expression for PbCl2 ⇌ Pb2+ + 2Cl-?

• Ksp = [Pb2+][Cl-]
• Ksp = [Pb2+][Cl-]^2
• Ksp = [Pb2+]^2[Cl-]
• Ksp = [Pb2+]^2[Cl-]^2

Correct Answer: Ksp = [Pb2+][Cl-]^2

Q8. Which factor will increase the solubility of AgCl in water?

• Adding NaCl to the solution
• Adding NH3 that forms Ag(NH3)2+
• Increasing Cl- concentration
• Decreasing temperature if dissolution is exothermic

Correct Answer: Adding NH3 that forms Ag(NH3)2+

Q9. What is the common-ion effect on the solubility of a salt?

• Solubility increases when a common ion is added
• Solubility decreases when a common ion is added
• Solubility is unchanged by a common ion
• Common ions convert salt into a complex which always increases solubility

Correct Answer: Solubility decreases when a common ion is added

Q10. How does decreasing pH affect the solubility of a salt containing a basic anion (e.g., CaF2)?

• Solubility decreases because H+ promotes precipitation
• Solubility increases because H+ reacts with the basic anion, shifting equilibrium toward dissolution
• pH has no effect on salts with fluoride or other basic anions
• Solubility only depends on temperature, not pH

Correct Answer: Solubility increases because H+ reacts with the basic anion, shifting equilibrium toward dissolution

Q11. Which statement best describes the role of complexation in solubility?

• Complexation always decreases solubility by stabilizing the solid lattice
• Complexation increases solubility by removing free ions and shifting the dissolution equilibrium
• Complexation has no effect on solubility
• Complexation converts ions into neutral molecules and causes precipitation

Correct Answer: Complexation increases solubility by removing free ions and shifting the dissolution equilibrium

Q12. If the ionic product Q for a solution is greater than Ksp, what will occur?

• The solution is unsaturated and no change occurs
• Precipitation will occur until Q equals Ksp
• The solution becomes more soluble indefinitely
• The solid dissolves further until Q equals Ksp

Correct Answer: Precipitation will occur until Q equals Ksp

Q13. Given [Ag+] = 1.0×10^-5 M and [Cl-] = 1.0×10^-4 M, and Ksp(AgCl) = 1.8×10^-10, will AgCl precipitate?

• No, because Q = 1.0×10^-9 which is less than Ksp
• Yes, because Q = 1.0×10^-9 which is greater than Ksp
• No, because Q = 1.8×10^-11 which is less than Ksp
• Yes, because Q = 1.0×10^-10 which equals Ksp

Correct Answer: Yes, because Q = 1.0×10^-9 which is greater than Ksp

Q14. If Ksp(AgCl) = 1.8×10^-10, what is the molar solubility s of AgCl in pure water (approximate)?

• s ≈ 1.3×10^-5 M
• s ≈ 1.8×10^-10 M
• s ≈ 4.2×10^-6 M
• s ≈ 2.7×10^-3 M

Correct Answer: s ≈ 1.3×10^-5 M

Q15. How does adding a soluble source of Ca2+ affect the solubility of CaF2?

• It increases solubility due to complexation with fluoride
• It decreases solubility due to the common-ion effect
• It has no effect because CaF2 solubility is pH-dependent only
• It converts CaF2 to a different solid phase increasing solubility

Correct Answer: It decreases solubility due to the common-ion effect

Q16. Which condition will increase the solubility of Al(OH)3 in water?

• Increasing pH to strongly alkaline
• Decreasing pH (making solution acidic)
• Adding a large concentration of OH-
• Adding inert salt to increase ionic strength only

Correct Answer: Decreasing pH (making solution acidic)

Q17. Among the following, which is the most effective way to increase the solubility of a sparingly soluble metal halide?

• Add a common halide ion
• Lower the temperature for endothermic dissolution
• Add a ligand that forms a strong complex with the metal ion
• Decrease solvent polarity

Correct Answer: Add a ligand that forms a strong complex with the metal ion

Q18. Given Ksp(AgCl) ≈ 1.8×10^-10 and Ksp(PbCl2) ≈ 1.7×10^-5, which chloride will precipitate first when Cl- is slowly added to a solution containing equal molar Ag+ and Pb2+?

• PbCl2 will precipitate first because its Ksp is larger
• AgCl will precipitate first because its Ksp is smaller
• Both will precipitate simultaneously
• Neither will precipitate because Ksp values are irrelevant

Correct Answer: AgCl will precipitate first because its Ksp is smaller

Q19. How does the solubility of an ionic solid change with temperature if the dissolution process is endothermic?

• Solubility decreases with increasing temperature
• Solubility increases with increasing temperature
• Solubility is independent of temperature
• Solubility first decreases then increases with temperature

Correct Answer: Solubility increases with increasing temperature

Q20. Which statement about the units of Ksp is correct?

• Ksp is always dimensionless
• Ksp always has units of mol/L
• Units of Ksp depend on the stoichiometry of the dissolution reaction
• Ksp units are always L^2 mol^-2

Correct Answer: Units of Ksp depend on the stoichiometry of the dissolution reaction

Q21. What is the Ksp expression for Mg(OH)2 ⇌ Mg2+ + 2OH-?

• Ksp = [Mg2+][OH-]
• Ksp = [Mg2+][OH-]^2
• Ksp = [Mg2+]^2[OH-]
• Ksp = [Mg2+]^2[OH-]^2

Correct Answer: Ksp = [Mg2+][OH-]^2

Q22. For CaSO4 dissolving as CaSO4 ⇌ Ca2+ + SO4^2-, if molar solubility is s, what is Ksp?

• Ksp = s^2
• Ksp = 4s^3
• Ksp = s
• Ksp = 2s^2

Correct Answer: Ksp = s^2

Q23. In a saturated solution of pure AgCl, what is the relationship between [Ag+] and [Cl-] from dissolution alone?

• [Ag+] = 2[Cl-]
• [Ag+] = [Cl-]
• [Ag+] = 0.5[Cl-]
• [Ag+] = [Cl-]^2

Correct Answer: [Ag+] = [Cl-]

Q24. For a salt A2B3 dissolving as 2A3+ + 3B2-, if molar solubility is s, which gives Ksp in terms of s?

• Ksp = (2s)^2(3s)^3 = 4s^2·27s^3
• Ksp = s^5
• Ksp = (2s)(3s) = 6s^2
• Ksp = (2s)^3(3s)^2

Correct Answer: Ksp = (2s)^2(3s)^3 = 4s^2·27s^3

Q25. Which will favor precipitation of Mg(OH)2 from a magnesium-containing solution?

• Adding HCl to the solution
• Adding NaOH to increase [OH-]
• Adding a strong complexing agent for Mg2+
• Lowering ionic strength to zero

Correct Answer: Adding NaOH to increase [OH-]

Q26. When are activity corrections most important in Ksp calculations?

• At very low ionic strengths (<0.001 M)
• At high ionic strengths where ionic interactions are significant
• Only for non-electrolyte solubility
• Never; activities are always equal to concentrations

Correct Answer: At high ionic strengths where ionic interactions are significant

Q27. How do you convert molar solubility (s, mol/L) to solubility in g/L?

• Multiply s by Avogadro’s number
• Multiply s by the molar mass of the salt
• Divide s by the molar mass
• Multiply s by 1000 only

Correct Answer: Multiply s by the molar mass of the salt

Q28. Which statement is true for salts with basic anions (e.g., CO3^2-, F-)?

• Their solubility decreases as pH decreases
• Their solubility increases as pH decreases due to protonation of the anion
• They are unaffected by pH because carbonate and fluoride are non-basic
• Their Ksp becomes a function of temperature only

Correct Answer: Their solubility increases as pH decreases due to protonation of the anion

Q29. If Ksp(CaF2) = 4.0×10^-11, what is the approximate molar solubility s? (Hint: Ksp = 4s^3)

• s ≈ 1.0×10^-4 M
• s ≈ 2.5×10^-4 M
• s ≈ 2.9×10^-3 M
• s ≈ 3.4×10^-5 M

Correct Answer: s ≈ 3.4×10^-5 M

Q30. How can Ksp be used practically in pharmaceutics?

• To predict drug dissolution and precipitation during formulation and storage
• Only to determine flavor of oral drugs
• To calculate boiling points of solutions
• It has no practical use in pharmaceutics

Correct Answer: To predict drug dissolution and precipitation during formulation and storage

Q31. If a metal ion forms a very stable complex with a ligand (large Kf), what is the expected effect on the metal salt solubility?

• Solubility decreases because the complex is insoluble
• Solubility increases because free metal ion concentration is lowered
• No change because complex formation does not affect equilibrium
• Solubility becomes zero

Correct Answer: Solubility increases because free metal ion concentration is lowered

Q32. How can a precipitated metal ion be redissolved in a laboratory separation?

• By adding a complexing or chelating agent to form soluble complexes
• By adding more precipitating reagent
• By cooling the solution rapidly
• By removing all water immediately

Correct Answer: By adding a complexing or chelating agent to form soluble complexes

Q33. Which condition describes a supersaturated solution with respect to a salt?

• Q < Ksp and no solid present
• Q = Ksp and equilibrium achieved
• Q > Ksp but no precipitation has yet occurred
• Q = 0 always

Correct Answer: Q > Ksp but no precipitation has yet occurred

Q34. Which variable has the least effect on the Ksp of an ionic solid in aqueous solution?

• Pressure (for a condensed solid and liquid)
• Temperature (if dissolution is endothermic/exothermic)
• Presence of common ions
• Complexing ligands in solution

Correct Answer: Pressure (for a condensed solid and liquid)

Q35. Which two thermodynamic factors primarily determine intrinsic solubility of an ionic solid?

• Lattice energy of the solid and hydration enthalpy of the ions
• Boiling point and melting point of the salt
• Color and crystal habit of the solid
• Viscosity and surface tension of the solvent

Correct Answer: Lattice energy of the solid and hydration enthalpy of the ions

Q36. For Ag2SO4 dissolving as 2Ag+ + SO4^2-, which relation between molar solubility s and Ksp is correct?

• Ksp = s^2
• Ksp = 4s^3
• Ksp = (2s)^2(s) = 4s^3
• Ksp = 2s^2

Correct Answer: Ksp = (2s)^2(s) = 4s^3

Q37. What principle allows selective precipitation of one ion from a mixture of metal ions?

• Choose a precipitating agent that forms the least soluble compound (lowest Ksp) with one ion
• Use a reagent that increases temperature only
• Always precipitate all ions simultaneously
• Selective precipitation is impossible because Ksp are all equal

Correct Answer: Choose a precipitating agent that forms the least soluble compound (lowest Ksp) with one ion

Q38. Which best describes a saturated solution at equilibrium with a solid salt?

• The concentrations of ions are constantly rising
• The rate of dissolution equals the rate of precipitation
• The solid continues to dissolve indefinitely
• The solution contains no dissolved ions

Correct Answer: The rate of dissolution equals the rate of precipitation

Q39. What happens to the solubility of BaSO4 if sulfate concentration is increased by adding Na2SO4?

• Solubility increases due to ionic strength
• Solubility decreases due to the common-ion effect
• Solubility is unchanged because BaSO4 is unaffected by Na+
• BaSO4 converts to a more soluble sulfate

Correct Answer: Solubility decreases due to the common-ion effect

Q40. Activity coefficients typically have what effect on the “effective” concentration of ions at moderate to high ionic strength?

• Activities are larger than concentrations (coefficients >1)
• Activities are equal to concentrations always
• Activities are smaller than concentrations (coefficients <1)
• Activities are unrelated to concentration

Correct Answer: Activities are smaller than concentrations (coefficients <1)

Q41. How does the stoichiometry of dissolution affect the exponent of the concentration terms in Ksp?

• Exponent equals the charge of each ion only
• Exponent equals the stoichiometric coefficient of that ion in the dissolution equation
• Exponent is always 1 regardless of stoichiometry
• Exponent is determined by temperature solely

Correct Answer: Exponent equals the stoichiometric coefficient of that ion in the dissolution equation

Q42. Is the numerical value of Ksp pathway-dependent (i.e., depends on how dissolution occurs)?

• Yes, it depends on the dissolution mechanism
• No, Ksp is a state function and depends only on the final equilibrium
• Yes, it depends on mixing speed
• Yes, it depends on the brand of chemical used

Correct Answer: No, Ksp is a state function and depends only on the final equilibrium

Q43. Which strategy is effective for increasing the solubility of a drug salt that hydrolyzes in water?

• Adjust the pH to favor hydrolysis products that keep the ion in solution
• Add a common ion to suppress dissolution
• Remove all ligands that can complex the drug ion
• Lower temperature drastically regardless of enthalpy

Correct Answer: Adjust the pH to favor hydrolysis products that keep the ion in solution

Q44. When two salts that share a common ion are mixed, what determines whether precipitation occurs?

• Only the total volume of the mixture
• Whether the ionic product Q of the common ion and counter-ion exceeds the relevant Ksp
• The color of the salts
• Precipitation is random and cannot be predicted

Correct Answer: Whether the ionic product Q of the common ion and counter-ion exceeds the relevant Ksp

Q45. Which of the following correctly predicts the solubility change when an inert salt (e.g., NaCl) is added to a solution of a sparingly soluble salt?

• Inert salt always increases solubility by common-ion effect
• Inert salt can slightly change solubility through ionic strength (salting in or out) but not by common-ion effect
• Inert salt has no effect on solubility under any conditions
• Inert salt always precipitates the sparingly soluble salt

Correct Answer: Inert salt can slightly change solubility through ionic strength (salting in or out) but not by common-ion effect

Q46. For a pharmaceutical formulation, why is knowledge of Ksp important when designing a suspension?

• To ensure correct color of the suspension
• To predict whether the active pharmaceutical ingredient will remain dissolved or precipitate under given conditions
• It only matters for gas solubility, not suspensions
• Ksp controls microbial growth in the suspension

Correct Answer: To predict whether the active pharmaceutical ingredient will remain dissolved or precipitate under given conditions

Q47. If you want to selectively precipitate Ba2+ from a mixture containing Ca2+ and Ba2+, which approach is correct?

• Add sulfate slowly until BaSO4 precipitates because BaSO4 has much lower Ksp than CaSO4
• Add chloride because BaCl2 is less soluble than CaCl2
• Add Na+ to increase ionic strength only
• Lower pH to precipitate both ions simultaneously as hydroxides

Correct Answer: Add sulfate slowly until BaSO4 precipitates because BaSO4 has much lower Ksp than CaSO4

Q48. Which description fits a saturated solution in terms of Q and Ksp?

• Q < Ksp
• Q = Ksp
• Q > Ksp
• Q is undefined

Correct Answer: Q = Ksp

Q49. How does formation of volatile species (e.g., H2S removal) affect solubility equilibria of sulfide salts?

• Removal of H2S gas shifts equilibrium to produce more sulfide and decreases solubility
• Removal of H2S increases solubility by shifting equilibrium to dissolve more solid
• Volatile species never affect solubility
• Only temperature affects sulfide solubility, not gas removal

Correct Answer: Removal of H2S increases solubility by shifting equilibrium to dissolve more solid

Q50. Which calculation is required to predict whether a precipitate will form when two laboratory solutions are mixed?

• Compute the ionic product Q after mixing and compare it with Ksp
• Measure the color change only
• Calculate the pH only because Ksp is irrelevant
• Determine the boiling point of the mixture

Correct Answer: Compute the ionic product Q after mixing and compare it with Ksp

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com