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Home › Chemistry › Percentage Composition

Percentage Composition — Chemistry Questions & Practice

Practise 16 Percentage Composition questions from Chemistry for competitive exams such as UPSC, Banking, SSC and Railway. Read the worked questions below, check every answer with an explanation, and track what you get wrong.

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Practice questions on Percentage Composition — with answers

Try each question first, then open the answer. Explanations are included.

1. What is the percentage by mass of Oxygen in Water (H2O)?

  1. A88.89%
  2. B98.89%
  3. C85.89%
  4. D78.89%
Show answer

Answer: A

% Oxygen = (mass of Oxygen in formula / molar mass of compound) x 100 = (16/18) x 100 = 88.89%.

2. What is the percentage by mass of Hydrogen in Water (H2O)?

  1. A14.11%
  2. B11.11%
  3. C6.11%
  4. D16.11%
Show answer

Answer: B

% Hydrogen = (mass of Hydrogen in formula / molar mass of compound) x 100 = (2/18) x 100 = 11.11%.

3. What is the percentage by mass of Oxygen in Carbon dioxide (CO2)?

  1. A69.73%
  2. B75.73%
  3. C80.73%
  4. D72.73%
Show answer

Answer: D

% Oxygen = (mass of Oxygen in formula / molar mass of compound) x 100 = (32/44) x 100 = 72.73%.

4. What is the percentage by mass of Carbon in Carbon dioxide (CO2)?

  1. A19.27%
  2. B27.27%
  3. C22.27%
  4. D35.27%
Show answer

Answer: B

% Carbon = (mass of Carbon in formula / molar mass of compound) x 100 = (12/44) x 100 = 27.27%.

5. What is the percentage by mass of Carbon in Methane (CH4)?

  1. A75.0%
  2. B78.0%
  3. C85.0%
  4. D72.0%
Show answer

Answer: A

% Carbon = (mass of Carbon in formula / molar mass of compound) x 100 = (12/16) x 100 = 75.0%.

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