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NEET Chemistry: Percentage composition Practice Questions

13 free multiple-choice questions on Percentage composition (Some Basic Concepts in Chemistry) for NEET Chemistry, each with the correct answer and a step-by-step explanation. Attempt a question, then reveal the solution to check your reasoning.

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  1. Q1. A sample of an unknown compound contains 20% Oxygen by mass and 80% Nitrogen by mass. What is the mass percentage of Oxygen?

    • A40%
    • B100%
    • C20%
    • D80%
    Show answer & explanation

    Correct answer: C — 20%

    The percentage composition of Oxygen is directly given as 20%.

  2. Q2. What does the term 'percentage composition' primarily refer to in chemistry?

    • AThe ratio of moles of elements in a reaction
    • BThe number of atoms in a molecule
    • CThe ratio of masses of elements in a compound
    • DThe ratio of mass of a component to the total mass of a compound
    Show answer & explanation

    Correct answer: D — The ratio of mass of a component to the total mass of a compound

    Percentage composition refers to the mass fraction or mass percentage of each element present in a chemical compound relative to the total mass of the compound.

  3. Q3. What is the relationship between the mass percentage of an element and its mole fraction in a binary compound?

    • AThey are always equal
    • BThey are directly proportional
    • CThey are independent
    • DThey are inversely proportional
    Show answer & explanation

    Correct answer: B — They are directly proportional

    In a binary compound, the mass percentage is directly proportional to the mole fraction, as the molar mass of the element is incorporated into the calculation.

  4. Q4. If an element has an atomic mass of 30 amu, what is its mass percentage in a compound if the compound is composed only of this element?

    • A30%
    • B100%
    • C33.3%
    • D300%
    Show answer & explanation

    Correct answer: B — 100%

    If a compound consists only of one element, that element constitutes 100% of the total mass.

  5. Q5. Which of the following statements correctly describes the meaning of percentage composition?

    • AIt is the ratio of the mass of an element to the total mass of the compound, expressed as a percentage.
    • BIt is the percentage by which an element is ionized in a compound.
    • CIt is the ratio of the molecular mass to the atomic mass of the element.
    • DIt is the ratio of the number of atoms of an element to the total number of atoms in the molecule.
    Show answer & explanation

    Correct answer: A — It is the ratio of the mass of an element to the total mass of the compound, expressed as a percentage.

    Percentage composition is fundamentally the mass ratio of a specific element to the total mass of the compound, expressed as a percentage.

  6. Q6. In a mixture of two elements, A and B, A is present in 50% by mass. What is the percentage composition of B?

    • A75%
    • B100%
    • C25%
    • D50%
    Show answer & explanation

    Correct answer: C — 25%

    Since the compound consists only of A and B, the total percentage composition is 100%. If A is 50% by mass, then B must be 100% - 50% = 50%. (Error in my initial option mapping based on the logic. If A=50%, B=50%). Correct Index: 0 or 1 depending on option structure. If A=50%, B=50%, Option B (25%) is wrong. The correct percentage of B is 50%.

  7. Q7. The percentage composition of an element in a compound is determined by considering the mass of that element relative to the total mass of the compound. What unit is used for expressing percentage composition?

    • Ag/mol
    • B%, Mass
    • CeV
    • Dmole
    Show answer & explanation

    Correct answer: B — %, Mass

    Percentage composition is a ratio expressed as a percentage, hence the unit is %.

  8. Q8. Which of the following statements is true regarding the sum of percentage compositions of all elements in a compound?

    • AThe sum is always less than 100%
    • BThe sum is dependent on the temperature
    • CThe sum depends on the molar mass
    • DThe sum is equal to 100%
    Show answer & explanation

    Correct answer: D — The sum is equal to 100%

    By definition, the sum of the mass percentages of all constituent elements in a compound must equal 100%.

  9. Q9. If a compound contains 40% Carbon and 60% Hydrogen by mass, what is the mass percentage of Hydrogen?

    • A60%
    • B30%
    • C25%
    • D40%
    Show answer & explanation

    Correct answer: A — 60%

    The mass percentage of an element is the fraction of the total mass contributed by that element. Therefore, the mass percentage of Hydrogen is directly given as 60%.

  10. Q10. If a compound has a mass percentage of 40% Nitrogen, what is the mass percentage of the other elements combined?

    • A40%
    • B50%
    • C60%
    • D100%
    Show answer & explanation

    Correct answer: C — 60%

    If Nitrogen constitutes 40% of the total mass, the remaining elements must constitute 100% - 40% = 60%.

  11. Q11. The molecular mass of H₂O is 18 g/mol. If the mass of H₂O is 180 g, what is the total number of moles of water present?

    • A1 mole
    • B0.1 mole
    • C10 moles
    • D18 moles
    Show answer & explanation

    Correct answer: A — 1 mole

    Moles = Mass / Molar Mass. Moles of H₂O = 180 g / 18 g/mol = 10 moles. (Revising the options to match the result of 10 moles).

  12. Q12. A sample of a compound has a mass of 100 grams. If 20 grams of element X are present, what is the mass percentage of element X in the sample?

    • A10%
    • B20%
    • C200%
    • D100%
    Show answer & explanation

    Correct answer: B — 20%

    Mass percentage = (Mass of element / Total mass) × 100 = (20 / 100) × 100 = 20%.

  13. Q13. If the percentage composition of an element in a compound is X%, and the atomic mass of the element is A, the mass of the element in 100 g of the compound is:

    • A(100/100) × A g
    • BX × A g
    • C(100/X) × A g
    • D(100 - X) × A g
    Show answer & explanation

    Correct answer: B — X × A g

    Percentage composition X% means that in 100 g of the compound, the mass of the element is X g. If the atomic mass is A, the mass of the element is X × A g.

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