Unit 9: Unit 9: Acid-Base Chemistry — Short Questions

Ans. (i) Ionization constant
It is the extent of ionization of acids and bases which is much less than 100%. It is the quantitative measure of the strength of an acid or base. 'Ka' is ionization constant of acids and 'Kb' is ionization constant of a bases.
HA + H₂O ⇌ H₃O⁺ + A⁻
Ka = [H₃O⁺][A⁻]/[HA]

(ii) Solubility product
It is the product of concentration of ions raised to the exponent equal to the coefficient of the balanced equation. It is represented by 'Ksp'
For example:
PbSO₄ ⇌ Pb²⁺ + SO₄²⁻
Ksp = [Pb²⁺][SO₄²⁻]/[PbSO₄]

(iii) Common ion effect
The suppression of ionization of a weak electrolyte by adding a common ion to it is called common ion effect.
For example:
When HCℓ gas is passed through the saturated solution of NaCℓ, due to formation of common Cℓ ions is increased and NaCℓ is crystallized out.
NaCℓ₍ₛ₎ ⇌ Na₍ₐq₎ + Cℓ₍ₐq₎
HCℓ₍ₐq₎ ⇌ H⁺₍ₐq₎ + Cℓ₍ₐq₎

(iv) Acid-base Indicator
An indicator is a substance that changes colour to mark a titration's and point. Acid base indicators exhibit one colour in acid and another in base. They are mostly weak organic acids or weak organic bases.
For example:
Phenolphthalein, methyl orange etc.

Ans. (i) Hydrolysis The process in which substance dissolves in water, dissociate into ions and interact with water molecules is called hydrolysis.
(i) It is a chemical process.
(ii) Formation of new substances occurs.
Example: Hydrolysis of ester
Dissolution
The process in which substance dissolves in solvent and form a homogeneous mixture.
(i) It is a physical process.
(ii) Formation of new substance does not occur
Example: Dissolution of sugar in water

(ii) Difference of acidic and basic buffer solution.
Acidic buffer: The buffer formed by mixing weak acid and salt of strong base is acidic buffer.
Example: Mixture of CH₃COOH and CH₃COONa
Basic Buffer: The buffer formed by mixing weak base and salt of strong acid is basic buffer
Example: Mixture of NH₃Cl and NH₄OH

Ans. Conjugate acid A specie formed when an acid donates a proton to a base is called conjugate acid.
Conjugate acid: A specie formed when an acid donates a proton to a base is called conjugate acid.
In this reaction, Cℓ⁻ and H₃O⁺ are conjugate acid- base pairs.
HCℓ + H₂O ⇌ H₃O⁺ + Cℓ⁻
acid base Conj.acid Conj.base

Ans. There is an inverse relationship between the strength of an acid and strength of its conjugate base.
The strong acid produces a weak conjugate base. Similarly, the strong base produces a weak conjugate acid.
Example: HCℓ strong acid and its conjugate base 'Cℓ⁻' is a weak base.

Ans. (i) NaOH₍ₛ₎ ⇌ Na₍ₐq₎ + OH⁻₍ₐq₎
In this reaction, NaOH dissociates in aqueous solution to produce OH⁻ ion. Hence, NaOH is an Arrhenius Base.

(ii) HF₍ₐq₎ + H₂O₍ₗ₎ ⇌ F⁻₍ₐq₎ + H₃O⁺₍ₐq₎
In this reaction, H₂O behaves as Bronsted-Lowry base because it accepts a proton (H⁺) from HF.

(iii) H⁺₍ₐq₎ + NH₃₍ₐq₎ ⇌ NH₄⁺₍ₐq₎
In this reaction, NH₃ behaves as Lewis base because it donates the pair of electrons to H⁺.

Ans. (i) H₂O+ HCℓ ⇌ H₃O⁺ + Cℓ⁻
In this reaction, H₂O behaves as a base because it accepts a proton (H⁺) from HCℓ.

(ii) NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
In this reaction, H₂O behaves as an acid because it donates a proton (H⁺) to NH₃.

(iii) HNO₃ + H₂O ⇌ H₃O⁺ + NO₃⁻
In this reaction, H₂O behave as a base because it accepts a proton (H⁺) from HNO₃.

(iv) CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
In this reaction, H₂O behave as a base because it accepts a proton (H⁺) from CH₃COOH.

Ans. Barium carbonate (BaCO₃) dissolves more readily than CuS in acidic solution.
When BaCO₃ dissociates.
BaCO₃ ⇌ Ba²⁺ + CO₃²⁻
Its conjugate base CO₃²⁻ reacts with H⁺ ions and form a weak acid BaCO₃.
CO₃²⁻ + 2H⁺ ⇌ H₂CO₃
Hence, due to formation of H₂CO₃, solubility of BaCO₃ increase.
While in CuS.
CuS ⇌ Cu²⁺ + S²⁻
Its conjugate base does not react with H⁺ ions. Hence, its solubility is less than BaCO₃.
More over the Ksp of BaCO₃ is more than the Ksp of CuS. So, BaCO₃ is more soluble than CuS.

Ans. The addition of a common ion to the solution of a less soluble electrolyte suppresses (decreases) its ionization and the concentration of unionized species increases, which may come out as precipitate. That's why solubility of salt decrease due to common ion effect.

Ans. The solubility product is the product of the concentrations of ions raised to an exponent equal to the co-efficient of the balanced equation.
Ksp is the measure of dissociation of sparingly soluble salt. It is the product of molar solubilities of two ions at equilibrium stage.
Factors affecting Ksp:
(i) Temperature - Ksp is temperature dependent value is usually very small at room temperature.
(ii) Common ion effect: The solubility of slightly soluble salts decreases due to common ions.

Ans. When the solution of Na₂CrO₄ and PbCrO₄ are mixed, the concentration of CrO₄²⁻₍ₐq₎ is increased according to Le-Chatelier's principle. The solubility of PbCrO₄ decreases as it precipitate out. This is due to common ion effect of CrO₄²⁻₍ₐq₎ ion.
PbCrO₄₍ₐq₎ ⇌ Pb²⁺₍ₐq₎ + CrO₄²⁻₍ₐq₎
Na₂CrO₄₍ₐq₎ ⇌ 2Na⁺ + CrO₄²⁻

Ans. Yes, according to the Lewis acid-base concept. Boron trifluoride (BF₃) can act as an acid. Because in BF₃, central atom 'B' has incomplete octet or deficit of 2 electrons and it has ability to accept an electron pair.

Ans. Solution [H⁺] = 1 × 10⁻⁵
pH = − log (1 × 10⁻⁵)
pH = 5

Ans. Acids
• They have sour taste.
• They change litmus from blue to red.
• They change the colour of dyes.
Base
• They have bitter taste.
• They change litmus from red to blue.
• They change colour of indicators like phenolphthalein from colourless to pink.

Ans. In Bronsted-Lowry theory, an acid is a proton (H⁺) donor, and a base is a proton acceptor. This concept is broader than Arrhenius.
Example: NH₃ + H₂O → NH₄⁺ + OH⁻

Ans. A conjugate acid-base pair differs by one proton. When an acid donates a proton, it forms its conjugate base, and vice versa.
Example: HCl / Cl⁻ and NH₄⁺ / NH₃ are conjugate pairs.

Ans. Amphoteric substances can act as both acids and bases depending on the environment.
Example: Water (H₂O) can accept or donate a proton.

Ans. Monoprotic acids donate one proton per molecule (e.g., HCl), while polyprotic acids donate more than one (e.g., H₂SO₄, H₃PO₄).

Ans. HCl donates H⁺ to NH₃:
HCl + NH₃ → NH₄Cl
It's a classic Bronsted acid-base reaction.

Ans. When a base accepts a proton, it becomes a conjugate acid. When an acid donates a proton, it forms a conjugate base.
Example: NH₃ → NH₄⁺; HCO₃⁻ → HCO₃⁻

Ans. Due to their chemical nature: acids release H⁺ which activate sour taste receptors, while bases release OH⁻ which give a slippery, bitter feel.

Ans. A Lewis acid is an electron pair acceptor, and a Lewis base is an electron pair donor
Example: NH₃ donates an electron pair to BF₃, which accepts it.

Ans. Ionization of water increases with temperature, so Kw and [H⁺], [OH⁻] increase, though pH may decrease slightly.

Ans. Kw = [H⁺][OH⁻] = 1×10⁻¹⁴ mol²/dm⁶ at 25°C. It reflects self-ionization of water.

Ans. pH is a measure of hydrogen ion concentration. It is calculated as:
pH = −log [H⁺]

Ans. A neutral solution has [H⁺] = 1×10⁻⁷ M, so pH = 7. Water is neutral at 25°C.

Ans. pOH = −log[OH⁻]. It is related to pH by the equation:
pH + pOH = 14 at 25°C.

Ans. Since HCl is a strong acid, it ionizes completely:
[H⁺] = 0.01 M
pH = −log(0.01) = 2

Ans. By acid dissociation constant Ka. Stronger base has higher Ka and produces more H⁺ ions.
The Ka is the ratio of concentration of acid ionized to the concentration of acid initially added.

Ans. pKa = −log Ka
Lower pKa means stronger acid. It's a logarithmic measure of acid strength.

Ans. Common Ion Effect
Definition: The suppression of ionization of a weak electrolyte by adding a common ion to it is called common ion effect.

Purification of sodium chloride NaCl is purified by passing hydrogen chloride gas through brine (saturated solution of NaCl). Sodium chloride is fully ionized in the solution.
Equilibrium constant expression for this process can be written as follows:
NaCl₍ₛ₎ ⇌ Na₍ₐq₎ + Cℓ₍ₐq₎

Ans. A buffer resists pH changes when small amounts of acid or base are added. It usually consists of a weak acid and its salt or weak base and its salt.

Ans. A mixture of CH₃COOH and CH₃COONa is an acidic buffer that maintains pH around 4.75.

Ans. Buffers are used in blood, pharmaceuticals, fermentation, and laboratory titrations to maintain stable pH.

Ans. Solubility Product
Definition: The solubility product is the product of the concentrations of ions raised to an exponent equal to the co-efficient of the balanced equation.
Example: The Ksp of PbSO₄ is 1.6 × 10⁻¹⁶

Ans. In salts of strong acids and strong bases like sodium chloride (NaCℓ).

The conjugate base of a strong acid (Cℓ⁻ from HCℓ) is very weak and does not significantly react with water.
The conjugate acid of a strong base (Na⁺ from NaOH) is also very weak and does not significantly react with water.
NaCℓ₍ₛ₎ ⇌ Na₍ₐq₎ + Cℓ₍ₐq₎

Na⁺ is the conjugate acid of NaOH (a strong base) and does not affect the pH.
Cℓ⁻ is the conjugate base of HCl (a strong acid) and does not affect the pH.

Ans. Salt hydrolysis
Definition: The breakdown of a salt (made up of weak acid or base) by reacting with water, resulting in the bond breaking in that salt which changes the pH of water is called hydrolysis.

Ans. Titration is a technique used to determine the concentration of an unknown acid or base using a solution of known concentration.

Ans. It is the point where moles of acid equal moles of base in titration. The pH at this point depends on the nature of acid/base.

Ans. Indicators change color near the equivalence point to help detect end of titration.
Example: Phenolphthalein turns pink in base.