Chemistry MCQs with answers Page - 137

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Q. Which of the following increases the rate of a chemical reaction according to collision theory?

• (A) Decreasing temperature
• (B) Reducing concentration of reactants
• (C) Adding a catalyst
• (D) Increasing activation energy

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Correct answer - Option (C)

Explanation:

A catalyst provides an alternative pathway with a lower activation energy, increasing the rate by increasing the fraction of collisions that are effective. Decreasing temperature lowers kinetic energy and reduces reaction rate (option1 wrong). Reducing concentration lowers collision frequency (option2 wrong). Increasing activation energy would decrease the rate (option4 wrong).

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Q. Which compound is amphoteric (can act as both an acid and a base)?

• (A) NaOH
• (B) HCl
• (C) Al(OH)3
• (D) CH4

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Correct answer - Option (C)

Explanation:

Aluminum hydroxide Al(OH)3 can react with acids (acting as a base) and with strong bases (acting as an acid) to form aluminate ions — that's amphoteric behavior. NaOH is purely basic, HCl is purely acidic, and CH4 (methane) is neither acidic nor basic in aqueous chemistry context. Therefore Al(OH)3 is correct.

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Q. What happens to the equilibrium constant (K) of an exothermic reaction if the temperature is increased?

• (A) K increases
• (B) K decreases
• (C) K remains unchanged
• (D) K becomes zero

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Correct answer - Option (B)

Explanation:

For an exothermic reaction (heat released as a product), increasing temperature shifts equilibrium toward reactants according to Le Chatelier, so the equilibrium constant K (product/reactant ratio at equilibrium) decreases. Option 1 is the opposite. Option 3 is false because K depends on temperature. Option 4 is impossible in normal chemical systems.

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Q. Which ion is responsible for the basicity of a solution when ammonia is dissolved in water?

• (A) NH4+
• (B) OH-
• (C) NH2-
• (D) H+

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Correct answer - Option (B)

Explanation:

When ammonia dissolves in water it reacts partially: NH3 + H2O ⇌ NH4+ + OH-. The OH- ion increases the pH, making the solution basic. NH4+ is the conjugate acid and its presence does not make the solution basic (it is slightly acidic). NH2- (amide) is not formed in aqueous ammonia under normal conditions. H+ would make solution acidic, not basic.

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Q. Which metric correctly describes the amount of substance containing Avogadro's number of entities?

• (A) One mole
• (B) One liter
• (C) One gram
• (D) One mole fraction

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Correct answer - Option (A)

Explanation:

A mole is defined as the amount of substance containing exactly Avogadro's number (6.022×10^23) of elementary entities (atoms, molecules, ions, etc.). One liter is a volume, one gram is a mass (depends on substance), and mole fraction is a ratio — none describe the specific count of entities like a mole does.

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Q. Which of the following elements has the highest electronegativity?

• (A) Oxygen
• (B) Fluorine
• (C) Nitrogen
• (D) Chlorine

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Correct answer - Option (B)

Explanation:

Fluorine is the most electronegative element in the periodic table (3.98 on the Pauling scale). It attracts electrons more strongly than oxygen, nitrogen, or chlorine. Hence, it is the correct answer.

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Q. What type of bond is formed between two chlorine atoms in Cl2?

• (A) Ionic bond
• (B) Hydrogen bond
• (C) Covalent bond
• (D) Metallic bond

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Correct answer - Option (C)

Explanation:

Each chlorine atom shares one electron to form a Cl–Cl single covalent bond. Ionic and metallic bonds involve electron transfer or delocalized electrons, which do not occur between two nonmetals like chlorine.

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Q. Which of the following acids is the strongest?

• (A) HCl
• (B) H2SO4
• (C) HNO3
• (D) H3PO4

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Correct answer - Option (A)

Explanation:

Among the given acids, HCl is a strong monoprotic acid that completely dissociates in water. While H2SO4 is also strong, only the first ionization is complete. HNO3 is strong but slightly less so than HCl, and H3PO4 is a weak acid.

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Q. Which gas is used in the preparation of soda water?

• (A) Carbon dioxide
• (B) Oxygen
• (C) Hydrogen
• (D) Nitrogen

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Correct answer - Option (A)

Explanation:

Soda water is prepared by dissolving carbon dioxide (CO2) gas under pressure in water. The resulting solution contains carbonic acid, giving it a tangy taste.

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Q. Which of the following metals is liquid at room temperature?

• (A) Mercury
• (B) Silver
• (C) Aluminum
• (D) Copper

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Correct answer - Option (A)

Explanation:

Mercury is the only metal that is liquid at room temperature (melting point -38.83°C). Other metals like silver, aluminum, and copper are solid at room temperature.

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