For known compounds also, such information provides a check whether the given sample contains the same percentage of elements as present in a pure sample. In other words, one can check the purity of a given sample by analysing this data.
-- NCERT Class 11 Chemistry, Ch. 1, p. 19Percentage Composition
8 MCQs9-step worked example
Source: NCERT Some Basic Concepts of ChemistryOfficial key: NTA-verifiedLast updated: 8 Oct 2026
Try this first
The mass percentage of an element in a compound is calculated as:
- A.(mass of the compound ÷ mass of the element) × 100
- B.(mass of the element ÷ mass of the other elements) × 100
- C.(number of atoms of the element ÷ total number of atoms) × 100
- D.(mass of that element in the compound ÷ molar mass of the compound) × 100
Tap to see the answer
Answer: D. Mass % of an element is the mass of that element in the compound divided by the molar mass of the compound, times 100 (NCERT Class 11 Chemistry, Chapter 1, page 19).
A is wrong: A is wrong because it inverts the ratio; that value is always above 100 (NCERT Class 11 Chemistry, Chapter 1, page 19).
B is wrong: B is wrong because the denominator must be the whole compound's molar mass, not only the other elements (NCERT Class 11 Chemistry, Chapter 1, page 19).
C is wrong: C is wrong because it counts atoms; mass percentage is based on mass, so heavy atoms weigh more than light ones (NCERT Class 11 Chemistry, Chapter 1, page 19).
Percentage Composition, explained for NEET
What fraction of water's mass is hydrogen: 2 of 3 atoms, or something much smaller? Mass percentage counts mass, not atoms. Hydrogen is two of the three atoms in H₂O but only about 11 % of its mass.
The rule. Mass % of an element = (mass of that element in the compound ÷ molar mass of the compound) × 100 (NCERT Class 11 Chemistry, Chapter 1, page 19). The denominator is the molar mass of the whole compound, and the mass of the element is its atomic mass times the number of its atoms in the formula.
NCERT's water example. With molar mass 18.02 g, oxygen is 16.00 ÷ 18.02 × 100 = 88.79 % and hydrogen is about 11.2 % (NCERT Class 11 Chemistry, Chapter 1, page 19). The page prints the hydrogen figure as 11.18; dividing 2.016 by 18.02 gives 11.19, so quote it as about 11.2 %.
NCERT's ethanol example. C₂H₅OH has molar mass 2 × 12.01 + 6 × 1.008 + 16.00 = 46.068 g; carbon is 24.02 ÷ 46.068 × 100 = 52.14 %, hydrogen 13.13 % and oxygen 34.73 % (NCERT Class 11 Chemistry, Chapter 1, page 19). Note that all six hydrogens in the formula count, including the one in OH.
Why it matters. The percentage composition of a known compound lets you check whether a sample contains the same percentage of elements as a pure sample, which is a check of its purity (NCERT Class 11 Chemistry, Chapter 1, page 19). If the mass percentages of a compound's elements are known, its empirical formula can be found (NCERT Class 11 Chemistry, Chapter 1, page 19).
Watch out: The percentages of all elements in one compound must add up to 100 (within rounding). If yours add to 100 only after you counted atoms instead of masses, the method is wrong and the match is accidental.
How do you solve a Percentage Composition question? A worked example
- 1
Given
Glucose, C₆H₁₂O₆. Atomic masses: C = 12.01, H = 1.008, O = 16.00 g/mol.
- 2
Required
The mass percentage of carbon, hydrogen and oxygen in glucose.
- 3
Concept
Percentage composition is the share of each element in the molar mass of the compound (NCERT Class 11 Chemistry, Chapter 1, page 19).
- 4
Formula
Mass % of an element = (mass of that element in the compound ÷ molar mass of the compound) × 100.
- 5
Substitution
Carbon: 6 × 12.01 = 72.06 g. Hydrogen: 12 × 1.008 = 12.096 g. Oxygen: 6 × 16.00 = 96.00 g. Molar mass = 72.06 + 12.096 + 96.00 = 180.156 g/mol.
- 6
Calculation
C = 72.06 ÷ 180.156 × 100 = 40.00 %. H = 12.096 ÷ 180.156 × 100 = 6.71 %. O = 96.00 ÷ 180.156 × 100 = 53.29 %. Check: 40.00 + 6.71 + 53.29 = 100.00. The subscripts 6 and 12 and the factor 100 are exact counting numbers and do not affect the significant-figure count.
- 7
Final answer
Glucose is 40.00 % carbon, 6.71 % hydrogen and 53.29 % oxygen by mass.
- 8
Common trap
Using atom fractions: glucose has 24 atoms and 12 of them are hydrogen, but hydrogen is only 6.71 % of the mass because each hydrogen atom is light.
- 9
Similar NEET-style question
What is the mass percentage of nitrogen in NH₄NO₃ (N = 14, H = 1, O = 16 g/mol, treated as exact)? (Answer: molar mass = 2 × 14 + 4 × 1 + 3 × 16 = 28 + 4 + 48 = 80 g/mol; N = 28 ÷ 80 × 100 = 35 %. Both nitrogens count: the one in NH₄ and the one in NO₃.)
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Can you answer these Percentage Composition MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
The mass percentage of an element in a compound is calculated as:
Show answer and why every option is right or wrong
Answer: D. Mass % of an element is the mass of that element in the compound divided by the molar mass of the compound, times 100 (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why A is wrong: A is wrong because it inverts the ratio; that value is always above 100 (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why B is wrong: B is wrong because the denominator must be the whole compound's molar mass, not only the other elements (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why C is wrong: C is wrong because it counts atoms; mass percentage is based on mass, so heavy atoms weigh more than light ones (NCERT Class 11 Chemistry, Chapter 1, page 19).
MCQ 2Easy RecallPractice
If the mass percentages of the elements in a compound are known, which quantity can be determined from them?
Show answer and why every option is right or wrong
Answer: B. NCERT states that if the mass per cent of the elements present in a compound is known, its empirical formula can be determined (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why A is wrong: A is wrong because the composition by mass does not give density (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why C is wrong: C is wrong because the melting point cannot be read from the percentages of the elements (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why D is wrong: D is wrong because isotopes do not follow from percentages of elements by mass (NCERT Class 11 Chemistry, Chapter 1, page 19).
MCQ 3Easy RecallPractice
In the calculation of the mass percentage of an element, the quantity in the denominator is:
Show answer and why every option is right or wrong
Answer: A. The denominator is the molar mass of the compound (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why B is wrong: B is wrong because dividing the element's mass by itself gives 100 % for every element (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why C is wrong: C is wrong because leaving out the element's own mass gives a ratio that does not describe the share of the compound (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why D is wrong: D is wrong because the atomic mass of the element sits in the numerator, multiplied by its number of atoms (NCERT Class 11 Chemistry, Chapter 1, page 19).
MCQ 4Direct ApplicationPractice
The molar mass of ethanol, C₂H₅OH, is 46.068 g/mol (C = 12.01, H = 1.008, O = 16.00 g/mol). The mass percentage of carbon in ethanol is:
Show answer and why every option is right or wrong
Answer: C. Two carbon atoms give 24.02 g, and 24.02 ÷ 46.068 × 100 = 52.14 % (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why A is wrong: A is wrong because 12.01 ÷ 46.068 uses one carbon atom; the formula has two (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why B is wrong: B is wrong because 34.73 % is the share of oxygen, 16.00 ÷ 46.068 × 100 (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why D is wrong: D is wrong because 13.13 % is the share of hydrogen, 6.048 ÷ 46.068 × 100 (NCERT Class 11 Chemistry, Chapter 1, page 19).
MCQ 5Direct ApplicationPractice
Take C = 12 and O = 16 g/mol (treated as exact). The mass percentage of oxygen in CO₂ is closest to:
Show answer and why every option is right or wrong
Answer: B. Molar mass of CO₂ = 12 + 2 × 16 = 44 g/mol; oxygen = 32 ÷ 44 × 100 = 72.73 % (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why A is wrong: A is wrong because 16 ÷ 44 × 100 counts only one oxygen atom; CO₂ has two (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why C is wrong: C is wrong because 12 ÷ 44 × 100 = 27.27 % is the share of carbon, not oxygen (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why D is wrong: D is wrong because 2 of 3 atoms is an atom fraction; the mass fraction weighs oxygen by its atomic mass (NCERT Class 11 Chemistry, Chapter 1, page 19).
MCQ 6Direct ApplicationPractice
Taking H = 1.008 g/mol and the molar mass of water as 18.02 g/mol, the mass percentage of hydrogen in H₂O is closest to:
Show answer and why every option is right or wrong
Answer: D. Two hydrogen atoms give 2.016 g, and 2.016 ÷ 18.02 × 100 ≈ 11.2 %; NCERT prints 11.18 for this figure, which is the same value to one decimal place (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why A is wrong: A is wrong because 1.008 ÷ 18.02 counts only one hydrogen atom; water has two (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why B is wrong: B is wrong because 88.8 % is the percentage of oxygen, 16.00 ÷ 18.02 × 100 (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why C is wrong: C is wrong because 2 of 3 atoms is an atom fraction, not a mass fraction (NCERT Class 11 Chemistry, Chapter 1, page 19).
MCQ 7CalculationPractice
Take Ca = 40, C = 12 and O = 16 g/mol (treated as exact). The mass of calcium in 125 g of pure CaCO₃ is:
Show answer and why every option is right or wrong
Answer: C. Molar mass of CaCO₃ = 40 + 12 + 3 × 16 = 100 g/mol, so calcium is 40 ÷ 100 × 100 = 40 %, and 0.40 × 125 g = 50.0 g (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why A is wrong: A is wrong because it quotes the percentage, 40, as grams without scaling to the 125 g sample (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why B is wrong: B is wrong because 60.0 g is the oxygen share of 125 g (48 % of 125 g); oxygen, not calcium, is meant (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why D is wrong: D is wrong because 75.0 g is 60 % of 125 g, the share of everything except calcium (NCERT Class 11 Chemistry, Chapter 1, page 19).
MCQ 8CalculationPractice
Take S = 32 and O = 16 g/mol (treated as exact). The mass of sulphur in 20.0 g of pure SO₃ is:
Show answer and why every option is right or wrong
Answer: A. Molar mass of SO₃ = 32 + 3 × 16 = 80 g/mol, so sulphur is 32 ÷ 80 = 40 %, and 0.40 × 20.0 g = 8.00 g (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why B is wrong: B is wrong because 12.0 g is the oxygen in the sample (60 % of 20.0 g), not the sulphur (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why C is wrong: C is wrong because 5.00 g takes sulphur as 1 of 4 atoms (25 %), an atom fraction instead of a mass fraction (NCERT Class 11 Chemistry, Chapter 1, page 19).
Why D is wrong: D is wrong because 13.3 g uses 32 ÷ 48, which treats the compound as SO with one oxygen atom instead of three (NCERT Class 11 Chemistry, Chapter 1, page 19).
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What to remember before solving Percentage Composition questions
4 NCERT lines
Mass % of an element = mass of that element in the compound 100 molar mass of th × e compound Molar mass of water = 18.02 g
-- NCERT Class 11 Chemistry, Ch. 1, p. 19Let us understand it by taking the example of water (H2O). Since water contains hydrogen and oxygen, the percentage composition of both these elements can be calculated as follows: Mass % of an element = mass of that element in the compound 100 molar mass of th × e compound Molar mass of water = 18.02 g Mass % of hydrogen = = 11.18 Mass % of oxygen = 16.00 18.02 100 × = 88.79
-- NCERT Class 11 Chemistry, Ch. 1, p. 19Let us take one more example. What is the percentage of carbon, hydrogen and oxygen in ethanol? Molecular formula of ethanol is: C2H5OH Molar mass of ethanol is: (2×12.01 + 6×1.008 + 16.00) g = 46.068 g Mass per cent of carbon = 24.02g 46.068g 100 × = 52.14% Mass per cent of hydrogen = 6.048g 46.068g 100 × = 13.13% Mass per cent of oxygen = 16.00g 46.068g 100 × = 34.73%
-- NCERT Class 11 Chemistry, Ch. 1, p. 19Percentage Composition: NEET previous year questions (PYQs) with answers
12 questions in Some Basic Concepts of Chemistry
No question in our NEET 2020–2025 set targets this topic directly.
All 12 past-paper questions from Some Basic Concepts of Chemistry →
More in Some Basic Concepts of Chemistry: 7 exam traps and mistakes · 3 formulas · 1 question pattern from its other lessons.
Sources
NCERT refs: Class 11 Chemistry Chapter 1, p.19
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
Every past-paper question from this chapter:
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All 11 NEET PYQs of NCERT Class 11 Chemistry Chapter 1, in page order (17 min)
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