MCQ 61
What is the main focus of stoichiometry in a chemical reaction?
A) Energy changes
B) Mass conservation
C) Reaction rates
D) Physical states
Answer: B) Mass conservation.
Explanation: Stoichiometry primarily focuses on the conservation of mass and the quantitative relationships between reactants and products.
MCQ 62
Which of the following correctly represents a decomposition reaction?
A) H₂ + O₂ → H₂O
B) 2H₂O → 2H₂ + O₂
C) N₂ + 3H₂ → 2NH₃
D) Ca + Cl₂ → CaCl₂
Answer: B) 2H₂O → 2H₂ + O₂.
Explanation: A decomposition reaction involves a single compound breaking down into two or more products.
MCQ 63
In a reaction, if 15 g of substance A produces 10 g of substance B, what is the mass of the leftover substance?
A) 5 g
B) 10 g
C) 15 g
D) 0 g
Answer: A) 5 g.
Explanation: The Law of Conservation of Mass implies that the mass before and after the reaction must equal; if 15 g produces 10 g, then 5 g remains.
MCQ 64
What does the term “molar mass” refer to?
A) The mass of a single atom
B) The mass of one mole of a substance
C) The volume of one mole of gas
D) The total mass of products
Answer: B) The mass of one mole of a substance.
Explanation: Molar mass is the mass of one mole of a substance, usually expressed in grams per mole (g/mol).
MCQ 65
Which of the following is NOT a characteristic of a chemical change?
A) Formation of a gas
B) Change in color
C) Change in state
D) Production of light
Answer: C) Change in state.
Explanation: A change in state (like melting or boiling) is a physical change, not necessarily a chemical change.
MCQ 66
What is the purpose of balancing a chemical equation?
A) To show the physical states of compounds
B) To ensure the same number of each type of atom on both sides
C) To represent the energy changes
D) To indicate the temperature of the reaction
Answer: B) To ensure the same number of each type of atom on both sides.
Explanation: Balancing ensures that the law of conservation of mass is followed by having equal numbers of each type of atom in reactants and products.
MCQ 67
Which compound has the smallest mass ratio of elements?
A) CO₂
B) H₂O
C) NH₃
D) CH₄
Answer: B) H₂O.
Explanation: The mass ratio of hydrogen to oxygen in water (1:8) is smaller than in the other compounds listed.
MCQ 68
What happens during a combustion reaction?
A) A compound breaks down into simpler substances.
B) Oxygen reacts with a substance, releasing energy.
C) Two elements combine to form a compound.
D) An acid reacts with a base.
Answer: B) Oxygen reacts with a substance, releasing energy.
Explanation: Combustion reactions typically involve a substance reacting with oxygen to produce heat and light.
MCQ 69
Which law explains the ratio of masses in the formation of compounds?
A) Law of Multiple Proportions
B) Law of Conservation of Mass
C) Law of Definite Proportions
D) Dalton’s Law
Answer: C) Law of Definite Proportions.
Explanation: This law states that a given compound always contains its constituent elements in a fixed mass ratio.
MCQ 70
In an exothermic reaction, what happens to the energy?
A) Energy is absorbed.
B) Energy is released.
C) Energy remains constant.
D) Energy is lost to the environment.
Answer: B) Energy is released.
Explanation: In an exothermic reaction, energy is released to the surroundings, usually in the form of heat.
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