Free Acids, Bases and Salts MCQs with Answers

11. According to the Bronsted-Lowry concept, which of the following species cannot function as an acid?

• A. SO₄ ²⁻
• B. H₃O⁺
• C. HSO₄⁻
• D. NH₄⁺

Explanation: A Bronsted-Lowry acid is a proton (H⁺) donor. The sulfate ion (SO₄²⁻) does not have any protons to donate, so it cannot function as a Bronsted-Lowry acid.

12. Compounds that tend to donate electron pairs are known as Lewis bases. Which one of the following is not a Lewis base?

• A. CH₃-NH₂
• B. PH₃
• C. AlCl₃
• D. H₂O

Explanation: A Lewis base has a lone pair of electrons to donate. Aluminum chloride (AlCl₃) is electron-deficient as the central aluminum atom has an incomplete octet. It acts as a Lewis acid by accepting an electron pair.

13. The smaller the value of pKb :

• A. The weaker the base
• B. The stronger the base
• C. The stronger the acid
• D. None of the above

Explanation: pKb is the negative logarithm of the base dissociation constant, K(-log Kb ). A smaller pKb value corresponds to a larger Kb value, which indicates a greater extent of dissociation and therefore a stronger base.

14. Which one of the following acids has the highest pH value?

• A. HCl(aq)
• B. HNO₃(aq)
• C. HF(aq)
• D. H₂SO₄(aq)

Explanation: pH is a measure of acidity; a higher pH means a weaker acid. Among the choices, hydrofluoric acid (HF) is the weakest acid. The others are strong acids that dissociate more completely, leading to lower pH values.

15. An acid is a substance which accepts:

• A. An electron pair
• B. a proton
• C. an electron
• D. a pair of protons

Explanation: A Lewis acid is defined as a substance that accepts an electron pair to form a coordinate bond. Since the question uses this definition. Thus, the correct option is A.

16. The colour of methyl orange in base is?

• A. Red
• B. Yellow
• C. Orange
• D. Pink

Explanation: Methyl orange is an acid-base indicator that is red in acidic medium. In basic solution, it changes to its yellow form. Thus, its colour in a base is yellow.

17. In the case of an acidic buffer solution, the concentration of the conjugate base is prominently supplied by the:

• A. Acid
• B. Salt
• C. Base
• D. Solvent

Explanation: An acidic buffer consists of a weak acid and its salt. The salt is a strong electrolyte that dissociates completely in solution, providing a large reservoir of the conjugate base.

18. A salt of a weak acid & a weak base is:

• A. NH₄NO₃
• B. NaCN
• C. NH₄NO₂
• D. K₂CO₃

Explanation: Ammonium nitrite (NH₄NO₂) is formed from the reaction of a weak base, ammonia (NH₃ or NH₄OH), and a weak acid, nitrous acid (HNO₂).

19. The aqueous solution of which of the following is acidic?

• A. CH₃COONa
• B. NaCN
• C. CuCl₂
• D. Na₂CO₃

Explanation: Copper(II) chloride (CuCl₂) is a salt of a weak base (Cu(OH)₂) and a strong acid (HCl). The Cu²⁺ ion hydrolyzes water to produce H⁺ ions, making the solution acidic.

20. Which of the following can form an acidic buffer?

• A. Acetic acid / sodium acetate
• B. Phosphoric acid / potassium dihydrogen phosphate
• C. Formic acid / sodium formate
• D. All of these

Explanation: An acidic buffer is a mixture of a weak acid and its conjugate base (usually from a salt). All three options consist of a weak acid and its corresponding salt, so all of them will form acidic buffer solutions.