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Class 11  »  Chemistry  »  Some Basic Concepts of Chemistry

Percentage Composition and Empirical Formula

Class 11

Chapter: Some Basic Concepts of Chemistry | Topic: Percentage Composition and Empirical Formula

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Class 11

PERCENTAGE COMPOSITION AND EMPIRICAL FORMULA

Topic: Percentage Composition and Empirical Formula

Time: 40 mins | Marks: 30 | Difficulty: Medium

Multiple Choice Questions

Q1. The empirical formula of a compound represents:

  1. The actual number of atoms of each element in a molecule
  2. The simplest whole number ratio of atoms of each element
  3. The molecular mass of the compound
  4. The percentage composition of the compound

Q2. The empirical formula of glucose (C₆H₁₂O₆) is:

  1. C₆H₁₂O₆
  2. CH₂O
  3. C₂H₄O₂
  4. C₃H₆O₃

Q3. If a compound contains 40% carbon, 6.67% hydrogen, and 53.33% oxygen by mass, its empirical formula is:

  1. CH₂O
  2. C₂H₄O₂
  3. CHO
  4. C₂H₂O₂

Q4. The molecular formula is related to the empirical formula by:

  1. Addition
  2. Subtraction
  3. Multiplication by a whole number (n)
  4. Division by a whole number

Q5. Percentage composition of an element in a compound is calculated as:

  1. (Mass of element in compound / Total mass of compound) × 100
  2. (Total mass of compound / Mass of element) × 100
  3. (Number of atoms of element / Total atoms) × 100
  4. (Molecular mass / Atomic mass) × 100

Q6. A compound has empirical formula CH₂O and molecular mass 180 g/mol. Its molecular formula is:

  1. CH₂O
  2. C₂H₄O₂
  3. C₆H₁₂O₆
  4. C₃H₆O₃

Short Answer Questions

Q7. Define empirical formula and molecular formula. How are they related?

Q8. Calculate the percentage of oxygen in water (H₂O).

Q9. What is percentage composition? Why is it useful in identifying an unknown compound?

Q10. A compound contains carbon and hydrogen only. Its empirical formula is CH₂. Can you determine its molecular formula without additional information? What else do you need?

Q11. Your mother makes a special masala for biryani using exactly 60% coriander, 20% cumin, and 20% chilli powder by weight. If she makes 500 g of this masala, how much of each ingredient does she use? How is this similar to calculating percentage composition in chemistry?

Q12. Why is the empirical formula of hydrogen peroxide (H₂O₂) written as HO, while its molecular formula remains H₂O₂?

Long Answer Questions

Q13. Explain the steps involved in determining the empirical formula of a compound from its percentage composition. Illustrate with an example where a compound contains 52.14% carbon, 13.13% hydrogen, and 34.73% oxygen by mass.

Q14. Describe how to determine the molecular formula of a compound when both the empirical formula and molecular mass are known. A compound has empirical formula CH₂O and molecular mass 60 g/mol. Determine its molecular formula. What additional information would you need if only the empirical formula was given?

Q15. A forensic chemist receives an unknown white powder from a crime scene. Analysis shows it contains 40.0% carbon, 6.67% hydrogen, and 53.33% oxygen by mass. The mass spectrometer indicates its molecular mass is approximately 180 g/mol.

(a) Calculate the empirical formula of the compound.

(b) Determine the molecular formula.

(c) The chemist suspects it might be glucose (found in sugar) or a toxic substance. How does knowing the molecular formula help narrow down the possibilities?

(d) If the compound were instead found to contain nitrogen as well, with composition 40% C, 6.67% H, 53.33% O, and no N, why is this analysis inconsistent with a protein-based poison?

Numerical / Application-Based Problems

Q16. A compound contains 43.64% phosphorus and 56.36% oxygen by mass.

(a) Calculate the empirical formula of the compound.

(b) If its molecular mass is 284 g/mol, determine its molecular formula.

(c) Name the compound.

(Atomic masses: P = 31 u, O = 16 u)

Q17. Urea (NH₂CONH₂) is an important nitrogenous fertilizer used by Indian farmers.

(a) Calculate the percentage composition of each element in urea.

(b) A farmer applies 50 kg of urea to his field. Calculate the mass of nitrogen actually added to the soil.

(c) If the farmer wants to supply exactly 20 kg of nitrogen, how much urea should he use?

(Atomic masses: N = 14 u, H = 1 u, C = 12 u, O = 16 u)

Q18. An organic compound on analysis gave the following data: C = 60%, H = 13.33%, O = 26.67%. Its vapour density is 30.

(a) Calculate the empirical formula of the compound.

(b) Calculate the molecular mass from vapour density.

(c) Determine the molecular formula.

(d) Suggest a possible compound that matches this formula.

(Atomic masses: C = 12 u, H = 1 u, O = 16 u) 


Total: 30 Marks | Time: 40 mins

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Importance and Scope of Chemistry Laws of Chemical Combination Daltons Atomic Theory Atomic and Molecular Mass Mole Concept and Molar Mass Stoichiometry and Stoichiometric Calculations Limiting Reagent Concentration Terms Molarity Molality Normality Mole Fraction