Chemistry MCQs with answers Page - 135

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Q. Which of the following is the purest form of Carbon ?

• (A) Charcoal
• (B) Granite
• (C) Diamond
• (D) Graphite

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Correct answer - Option (C)

Explanation:

Diamond is the purest form of carbon or diamond is 100% carbon. It is made from graphite in the presence of catalysts such as iron, cobalt, chromium, or manganese. It is considered to be the purest form of carbon, because the composition of the diamond is homogeneous

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Q. Which gas is produced when dilute hydrochloric acid reacts with zinc?

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

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Correct answer - Option (A)

Explanation:

Zinc reacts with dilute HCl in a single displacement reaction: Zn + 2HCl → ZnCl2 + H2. Hydrogen gas is liberated. Oxygen is not produced because this is not a decomposition or combustion reaction. Chlorine would require oxidation of chloride ions; instead chloride remains as Cl- in ZnCl2. Carbon dioxide is produced when carbonates react with acids, not metals reacting with acids.

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Q. What is the pH of a 0.001 M solution of HCl (a strong acid)?

• (A) 3
• (B) 11
• (C) 1
• (D) 7

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Correct answer - Option (A)

Explanation:

HCl is a strong acid and fully dissociates: [H+] = 0.001 M = 1 × 10^-3 M. pH = -log[H+] = -log(1×10^-3) = 3. Option 11 corresponds to a very basic solution, not acidic. pH 1 would correspond to 0.1 M H+, and pH 7 is neutral (pure water), not for a 0.001 M strong acid.

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Q. Which hybridization is present on the carbon atoms of ethene (C2H4)?

• (A) sp3
• (B) sp
• (C) sp2
• (D) dsp2

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Correct answer - Option (C)

Explanation:

In ethene each carbon forms three sigma bonds (two C-H and one C-C) and one pi bond (C=C), which corresponds to sp2 hybridization (three hybrid orbitals and one unhybridized p orbital for the pi bond). sp3 implies four sigma bonds (single bonds only) — incorrect. sp corresponds to linear geometry with two pi bonds (as in acetylene) — incorrect. dsp2 is used in some transition-metal complexes, not in simple organic carbon atoms.

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Q. For the reaction 2A → products, the rate law is rate = k[A]^2. What is the order of the reaction with respect to A and overall?

• (A) First order with respect to A; overall first order
• (B) Second order with respect to A; overall second order
• (C) Zero order with respect to A; overall zero order
• (D) Second order with respect to A; overall first order

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Correct answer - Option (B)

Explanation:

The exponent of [A] in the rate law gives the order with respect to A: it's 2, so second order in A. Since there's only A in the rate law, the overall order is also 2. Option 1 is wrong because the exponent is not 1. Option 3 is wrong because zero order would have [A]^0. Option 4 is inconsistent (can't be second in A and overall first).

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Q. Which of the following salts will produce a basic solution when dissolved in water?

• (A) NH4Cl
• (B) NaCl
• (C) Na2CO3
• (D) KNO3

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Correct answer - Option (C)

Explanation:

Na2CO3 (sodium carbonate) contains the carbonate ion CO3^2-, which is the conjugate base of the weak acid HCO3^- / H2CO3, so it hydrolyzes to produce OH- leading to a basic solution. NH4Cl contains NH4+ (acidic) and Cl- (neutral) so the solution is acidic. NaCl and KNO3 are salts of strong acids and strong bases (neutral ions) and produce neutral solutions (approximately pH 7).

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Q. Which statement best describes Le Chatelier's principle?

• (A) A system at equilibrium will shift to counteract any imposed change in conditions.
• (B) Equilibrium constants change when the concentration changes.
• (C) Adding inert gas to a reaction mixture at constant volume shifts the equilibrium position.
• (D) Temperature has no effect on equilibrium position.

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Correct answer - Option (A)

Explanation:

Le Chatelier's principle states that an equilibrium system responds to disturbances (changes in concentration, pressure, temperature) by shifting to oppose the change. Option 2 is false because the equilibrium constant K remains the same for a given temperature; only changing temperature changes K. Option 3 is generally false for constant volume — adding an inert gas at constant volume does not change partial pressures of reactants/products, so equilibrium doesn't shift. Option 4 is false because temperature changes do affect equilibrium constants and positions.

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Q. Which is the best oxidizing agent among the following?

• (A) F2
• (B) Cl2
• (C) Br2
• (D) I2

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Correct answer - Option (A)

Explanation:

Fluorine (F2) is the most electronegative element and the strongest oxidizing agent among the halogens — it readily accepts electrons. Chlorine, bromine, and iodine are progressively weaker oxidizers down the group. Therefore F2 > Cl2 > Br2 > I2. The other options are weaker oxidizing agents compared to F2.

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Q. Which of the following is NOT a colligative property?

• (A) Boiling point elevation
• (B) Vapor pressure lowering
• (C) Osmotic pressure
• (D) Color change

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Correct answer - Option (D)

Explanation:

Colligative properties depend only on the number of solute particles, not their identity: boiling point elevation, vapor pressure lowering, freezing point depression, and osmotic pressure are colligative. Color change depends on the chemical identity (electronic transitions) of substances and is not colligative.

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Q. In an ideal gas, which of the following is true at the same temperature?

• (A) All gases have the same density.
• (B) All gases have the same average kinetic energy per molecule.
• (C) All gases have the same molar mass.
• (D) All gases have the same number of molecules in any given volume.

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Correct answer - Option (B)

Explanation:

At a given temperature, the average translational kinetic energy per molecule depends only on temperature: (1/2)mv^2_avg ∝ kT. Different gases have different molar masses and densities, so options 1 and 3 are false. Option 4 is false because number of molecules in a given volume depends on pressure and temperature; at the same T and P equal volumes contain same number of moles (Avogadro's law) but the statement lacked pressure and is ambiguous — moreover molar masses differ so mass density changes.

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