Chem-1: Some Basic Concepts of Chemistry30 Questions
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1. The branch of science dealing with the composition, structure and properties of matter is called:
A. Chemistry
B. Physics
C. Biology
D. Geology
Answer: A. Chemistry
Explanation: Chemistry is defined as the study of matter, its composition, structure, properties and the changes it undergoes.
2. Matter that has a definite composition and distinct properties is called a:
A. Pure substance
B. Mixture
C. Solution only
D. Colloid only
Answer: A. Pure substance
Explanation: A pure substance has a fixed, uniform composition, distinguishing it from mixtures which can vary in composition.
3. A substance made up of two or more elements chemically combined in a fixed ratio is called a:
A. Compound
B. Mixture
C. Element
D. Solution
Answer: A. Compound
Explanation: Compounds consist of two or more elements combined chemically in a definite, fixed proportion, unlike mixtures.
4. A pure substance that cannot be broken down into simpler substances by chemical means is called an:
A. Element
B. Compound
C. Mixture
D. Solution
Answer: A. Element
Explanation: Elements are the most basic pure substances, incapable of being decomposed further through ordinary chemical reactions.
5. The Law of Conservation of Mass states that mass can neither be created nor destroyed in a:
A. Chemical reaction
B. Physical change only
C. Nuclear reaction only
D. Biological process only
Answer: A. Chemical reaction
Explanation: This fundamental law states that the total mass of reactants equals the total mass of products in any chemical reaction.
6. The Law of Definite Proportions states that a given compound always contains the same elements in the same:
A. Proportion by mass
B. Proportion by volume only
C. Number of atoms only
D. Physical state
Answer: A. Proportion by mass
Explanation: This law states that regardless of the source, a pure compound always contains its constituent elements in a fixed mass ratio.
7. The SI unit for the amount of substance is the:
A. Mole
B. Gram
C. Litre
D. Kilogram
Answer: A. Mole
Explanation: The mole is the SI base unit used to express the amount of a substance, specifically the number of particles present.
8. The number of particles present in one mole of a substance is called:
A. Avogadro's number
B. Molar mass
C. Atomic mass
D. Molecular mass
Answer: A. Avogadro's number
Explanation: Avogadro's number, approximately 6.022 × 10²³, represents the fixed number of particles contained in one mole of any substance.
9. The approximate value of Avogadro's number is:
A. 6.022 × 10²³
B. 3.14 × 10⁸
C. 9.8 × 10²
D. 1.6 × 10⁻¹⁹
Answer: A. 6.022 × 10²³
Explanation: Avogadro's number is defined as approximately 6.022 × 10²³ particles (atoms, molecules, or ions) per mole.
10. The mass of one mole of a substance, expressed in grams, is called its:
A. Molar mass
B. Atomic number
C. Density
D. Volume
Answer: A. Molar mass
Explanation: Molar mass represents the mass in grams of exactly one mole of a given substance, numerically equal to its atomic or molecular mass in amu.
11. The relative mass of an atom compared to 1/12th the mass of a carbon-12 atom is called its:
A. Atomic mass
B. Molecular mass
C. Molar volume
D. Equivalent weight
Answer: A. Atomic mass
Explanation: Atomic mass is defined relative to the standard reference of carbon-12, specifically 1/12th of its mass.
12. The sum of the atomic masses of all atoms present in a molecule is called its:
A. Molecular mass
B. Atomic mass
C. Molar volume
D. Formula unit mass only
Answer: A. Molecular mass
Explanation: Molecular mass is calculated by summing the atomic masses of every atom present within a single molecule of the substance.
13. The number of moles of solute dissolved per litre of solution is called its:
A. Molarity
B. Molality
C. Mole fraction
D. Mass percentage
Answer: A. Molarity
Explanation: Molarity specifically measures the concentration of a solution in terms of moles of solute per litre of the total solution.
14. The number of moles of solute per kilogram of solvent is called the solution's:
A. Molality
B. Molarity
C. Normality
D. Density
Answer: A. Molality
Explanation: Molality is defined using the mass of solvent (in kg) rather than the volume of solution, unlike molarity.
15. The ratio of the number of moles of a component to the total number of moles in a mixture is called its:
A. Mole fraction
B. Molarity
C. Molality
D. Mass fraction
Answer: A. Mole fraction
Explanation: Mole fraction expresses the proportion of moles of one component relative to the total moles present in the entire mixture or solution.
16. The percentage composition of a compound refers to the percentage by mass of each:
A. Element present in the compound
B. Molecule present only
C. Isotope present only
D. Ion present only
Answer: A. Element present in the compound
Explanation: Percentage composition specifically indicates the mass percentage contributed by each constituent element within the compound.
17. The simplest whole-number ratio of atoms of each element present in a compound is called its:
A. Empirical formula
B. Molecular formula
C. Structural formula
D. Ionic formula
Answer: A. Empirical formula
Explanation: The empirical formula represents the simplest ratio of elements in a compound, which may differ from the actual molecular formula.
18. The actual number of atoms of each element present in one molecule of a compound is given by its:
A. Molecular formula
B. Empirical formula
C. Structural formula only
D. Ionic formula only
Answer: A. Molecular formula
Explanation: The molecular formula specifies the exact, actual number of atoms of each element in a single molecule of the compound.
19. In a chemical reaction, the reactant that is completely consumed and limits the amount of product formed is called the:
A. Limiting reagent
B. Excess reagent
C. Catalyst
D. Product
Answer: A. Limiting reagent
Explanation: The limiting reagent is the reactant that runs out first, thereby determining the maximum possible amount of product that can be formed.
20. The theoretical yield of a chemical reaction refers to the:
A. Maximum amount of product predicted by stoichiometric calculation
B. Actual amount of product obtained in the lab
C. Amount of unreacted reactant remaining
D. Percentage error in measurement
Answer: A. Maximum amount of product predicted by stoichiometric calculation
Explanation: Theoretical yield represents the maximum possible amount of product calculated based on stoichiometry, assuming complete conversion of the limiting reagent.
21. The ratio of actual yield to theoretical yield, expressed as a percentage, is called the:
A. Percentage yield
B. Molar yield
C. Conversion factor
D. Stoichiometric ratio
Answer: A. Percentage yield
Explanation: Percentage yield indicates the efficiency of a reaction by comparing the actual (experimentally obtained) yield to the theoretical maximum.
22. The branch of chemistry dealing with the quantitative relationships between reactants and products is called:
A. Stoichiometry
B. Thermodynamics
C. Kinetics
D. Electrochemistry
Answer: A. Stoichiometry
Explanation: Stoichiometry specifically deals with calculating the quantities of reactants consumed and products formed in a chemical reaction.
23. According to Dalton's Atomic Theory, matter is composed of indivisible particles called:
A. Atoms
B. Molecules only
C. Ions only
D. Electrons only
Answer: A. Atoms
Explanation: Dalton's Atomic Theory proposed that all matter consists of extremely small, indivisible particles called atoms.
24. One mole of any gas at Standard Temperature and Pressure (STP) occupies a volume of approximately:
A. 22.4 litres
B. 1 litre
C. 100 litres
D. 6.022 litres
Answer: A. 22.4 litres
Explanation: According to the molar volume concept, one mole of any ideal gas occupies approximately 22.4 litres at STP conditions.
25. The Law of Multiple Proportions applies when two elements combine to form more than one compound, with the masses of one element combining with a fixed mass of the other in a ratio of:
A. Small whole numbers
B. Random decimals
C. Equal amounts always
D. Non-integer fractions only
Answer: A. Small whole numbers
Explanation: This law describes how, when forming multiple compounds, the combining masses of one element with a fixed mass of another form simple whole-number ratios.
26. Significant figures in a calculated chemical quantity reflect the:
A. Precision of the measured data used in the calculation
B. Exact theoretical value only
C. Random rounding choice
D. Colour of the chemical
Answer: A. Precision of the measured data used in the calculation
Explanation: The number of significant figures retained in a final calculated result should reflect the precision of the original experimental measurements.
27. The molar mass of water (H₂O) is approximately:
A. 18 g/mol
B. 16 g/mol
C. 2 g/mol
D. 36 g/mol
Answer: A. 18 g/mol
Explanation: Molar mass of H₂O = 2(1) + 16 = 18 g/mol, using approximate atomic masses of hydrogen (1) and oxygen (16).
28. If the empirical formula of a compound is CH₂O and its molecular mass is 180, its molecular formula is:
A. C₆H₁₂O₆
B. CH₂O
C. C₂H₄O₂
D. C₃H₆O₃
Answer: A. C₆H₁₂O₆
Explanation: Since the empirical formula mass of CH₂O is 30, and 180/30 = 6, the molecular formula is (CH₂O)₆ = C₆H₁₂O₆.
29. In balancing chemical equations, the Law of Conservation of Mass requires that the number of atoms of each element be:
A. Equal on both sides of the equation
B. Different on each side
C. Always doubled on the product side
D. Ignored for gaseous reactants
Answer: A. Equal on both sides of the equation
Explanation: To conserve mass, a balanced chemical equation must have an equal number of atoms of each element on both the reactant and product sides.
30. The concept of 'mole' in chemistry serves primarily to:
A. Relate the microscopic scale of atoms/molecules to macroscopic, measurable quantities
B. Measure temperature only
C. Measure only gaseous volumes
D. Replace the concept of mass entirely
Answer: A. Relate the microscopic scale of atoms/molecules to macroscopic, measurable quantities
Explanation: The mole concept provides a crucial bridge, allowing chemists to relate the incredibly small scale of individual atoms and molecules to measurable, everyday quantities like grams.
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