MCQ 21
Which of the following is an example of a chemical change?
A) Melting of ice
B) Dissolving sugar in water
C) Rusting of iron
D) Boiling of water
Answer: C) Rusting of iron.
Explanation: Rusting is a chemical change as it involves a reaction between iron and oxygen, producing rust (iron oxide).
MCQ 22
In the chemical formula for water (H₂O), what does the “2” represent?
A) The total number of atoms
B) The number of oxygen atoms
C) The number of hydrogen atoms
D) The ratio of hydrogen to oxygen
Answer: C) The number of hydrogen atoms.
Explanation: The “2” indicates that there are two hydrogen atoms for every one oxygen atom in a water molecule.
MCQ 23
According to Dalton’s Atomic Theory, which of the following is true about atoms?
A) Atoms can be created and destroyed.
B) Atoms of the same element are identical in mass and properties.
C) Atoms are divisible into smaller particles.
D) Atoms can exist in various states of matter.
Answer: B) Atoms of the same element are identical in mass and properties.
Explanation: Dalton stated that all atoms of a given element are identical, which is a key component of his atomic theory.
MCQ 24
What is the molar mass of water (H₂O)?
A) 18 g/mol
B) 16 g/mol
C) 20 g/mol
D) 10 g/mol
Answer: A) 18 g/mol.
Explanation: The molar mass of water is calculated as 2 (H) + 1 (O) = 2(1) + 16 = 18 g/mol.
MCQ 25
Which law explains why chemical formulas for compounds must reflect the actual composition?
A) Law of Definite Proportions
B) Law of Conservation of Mass
C) Law of Multiple Proportions
D) Dalton’s Law
Answer: A) Law of Definite Proportions.
Explanation: This law states that a chemical compound always contains its constituent elements in fixed proportions by mass, guiding the formulation of chemical formulas.
MCQ 26
Which of the following compounds is formed by the Law of Multiple Proportions?
A) H₂O and H₂O₂
B) CO and CO₂
C) NaCl and Na₂S
D) Both A and B
Answer: D) Both A and B.
Explanation: Both pairs of compounds are formed by two elements that can combine in different ratios to form different compounds.
MCQ 27
What is the primary focus of the Law of Conservation of Mass?
A) Energy changes in reactions
B) The balance of elements in compounds
C) The mass of reactants and products
D) The physical states of matter
Answer: C) The mass of reactants and products.
Explanation: This law specifically states that the mass remains constant during a chemical reaction, focusing on the total mass of reactants and products.
MCQ 28
If 6 g of carbon reacts with 16 g of oxygen to form carbon dioxide, what is the mass of carbon dioxide produced?
A) 10 g
B) 16 g
C) 22 g
D) 0 g
Answer: C) 22 g.
Explanation: The mass of carbon dioxide produced is equal to the total mass of reactants: 6 g + 16 g = 22 g.
MCQ 29
According to the Law of Definite Proportions, which of the following is a true statement?
A) H₂ and O₂ combine in a 2:1 ratio to form water.
B) Water can be made from different ratios of hydrogen and oxygen.
C) A given compound always contains the same proportion of elements by mass.
D) All compounds have the same chemical properties.
Answer: C) A given compound always contains the same proportion of elements by mass.
Explanation: This law emphasizes that the composition of a chemical compound is constant, regardless of its source.
MCQ 30
Which of the following reactions demonstrates the Law of Conservation of Mass?
A) Synthesis of water from hydrogen and oxygen
B) Decomposition of water into hydrogen and oxygen
C) Combustion of methane
D) All of the above
Answer: D) All of the above.
Explanation: Each of these reactions adheres to the Law of Conservation of Mass, where the total mass of reactants equals the total mass of products.
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