The Mole Concept - Percentage Composition

 The percentage composition in the context of the mole concept refers to the relative mass contributions of individual elements in a compound. It is expressed as the percentage of the total mass of a compound that each element contributes. The mole concept is a fundamental idea in chemistry that allows us to relate the mass of a substance to the number of entities (atoms, molecules, or ions) it contains.

Here's how you can calculate the percentage composition of a compound:

  1. Determine the Formula Mass: For a molecular compound, add up the atomic masses of all the atoms in a molecule. For an ionic compound, sum the atomic masses of all atoms in the formula unit.

  2. Calculate the Percentage Composition: For each element in the compound, divide the mass contributed by that element by the total formula mass, and multiply by 100 to get the percentage.

Let's take an example to illustrate:

Example: Water (H₂O)

  1. Determine the Formula Mass:

    • Atomic mass of hydrogen (H) = 1.01 g/mol (there are two hydrogen atoms)
    • Atomic mass of oxygen (O) = 16.00 g/mol
    • Formula mass of water = 2(1.01 g/mol) + 16.00 g/mol = 18.02 g/mol

  2. Calculate the Percentage Composition:

    • For hydrogen: (2 * 1.01 g/mol) / 18.02 g/mol * 100% ≈ 11.19%
    • For oxygen: (16.00 g/mol) / 18.02 g/mol * 100% ≈ 88.81%

So, the percentage composition of water is approximately 11.19% hydrogen and 88.81% oxygen.

This information is crucial because it helps chemists understand the composition of a substance and can be used to predict and verify experimental results. The mole concept allows for the conversion between mass and the number of entities, providing a bridge between the macroscopic world of grams and the microscopic world of atoms and molecules.

Certainly! Here are 20 questions on the percentage composition and mole concept, along with answers and explanations:

1. Question: Calculate the percentage composition of carbon in carbon dioxide (CO2).

Answer:

  • Carbon atomic mass = 12.01 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of CO2 = 12.01 + 2(16.00) = 44.01 g/mol
  • Percentage composition of carbon = (12.01 g/mol / 44.01 g/mol) * 100% ≈ 27.28%

2. Question: Find the percentage composition of hydrogen in methane (CH4).

Answer:

  • Hydrogen atomic mass = 1.01 g/mol
  • Carbon atomic mass = 12.01 g/mol
  • Formula mass of CH4 = 4(1.01) + 12.01 = 16.05 g/mol
  • Percentage composition of hydrogen = (4 * 1.01 g/mol / 16.05 g/mol) * 100% ≈ 25.16%

3. Question: Determine the percentage composition of oxygen in water (H2O).

Answer:

  • Hydrogen atomic mass = 1.01 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of H2O = 2(1.01) + 16.00 = 18.02 g/mol
  • Percentage composition of oxygen = (16.00 g/mol / 18.02 g/mol) * 100% ≈ 88.81%

4. Question: Calculate the percentage composition of nitrogen in ammonia (NH3).

Answer:

  • Nitrogen atomic mass = 14.01 g/mol
  • Hydrogen atomic mass = 1.01 g/mol
  • Formula mass of NH3 = 14.01 + 3(1.01) = 17.04 g/mol
  • Percentage composition of nitrogen = (14.01 g/mol / 17.04 g/mol) * 100% ≈ 82.05%

5. Question: Find the percentage composition of chlorine in hydrochloric acid (HCl).

Answer:

  • Hydrogen atomic mass = 1.01 g/mol
  • Chlorine atomic mass = 35.45 g/mol
  • Formula mass of HCl = 1.01 + 35.45 = 36.46 g/mol
  • Percentage composition of chlorine = (35.45 g/mol / 36.46 g/mol) * 100% ≈ 97.22%

6. Question: Determine the percentage composition of sulfur in hydrogen sulfide (H2S).

Answer:

  • Hydrogen atomic mass = 1.01 g/mol
  • Sulfur atomic mass = 32.07 g/mol
  • Formula mass of H2S = 2(1.01) + 32.07 = 34.09 g/mol
  • Percentage composition of sulfur = (32.07 g/mol / 34.09 g/mol) * 100% ≈ 94.25%

7. Question: Calculate the percentage composition of potassium in potassium chloride (KCl).

Answer:

  • Potassium atomic mass = 39.10 g/mol
  • Chlorine atomic mass = 35.45 g/mol
  • Formula mass of KCl = 39.10 + 35.45 = 74.55 g/mol
  • Percentage composition of potassium = (39.10 g/mol / 74.55 g/mol) * 100% ≈ 52.42%

8. Question: Find the percentage composition of magnesium in magnesium oxide (MgO).

Answer:

  • Magnesium atomic mass = 24.31 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of MgO = 24.31 + 16.00 = 40.31 g/mol
  • Percentage composition of magnesium = (24.31 g/mol / 40.31 g/mol) * 100% ≈ 60.36%

9. Question: Determine the percentage composition of carbon in ethane (C2H6).

Answer:

  • Carbon atomic mass = 12.01 g/mol
  • Hydrogen atomic mass = 1.01 g/mol
  • Formula mass of C2H6 = 2(12.01) + 6(1.01) = 30.07 g/mol
  • Percentage composition of carbon = (2 * 12.01 g/mol / 30.07 g/mol) * 100% ≈ 79.96%

10. Question: Calculate the percentage composition of oxygen in carbon monoxide (CO).

Answer:

  • Carbon atomic mass = 12.01 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of CO = 12.01 + 16.00 = 28.01 g/mol
  • Percentage composition of oxygen = (16.00 g/mol / 28.01 g/mol) * 100% ≈ 57.13%

11. Question: Find the percentage composition of nitrogen in nitric acid (HNO3).

Answer:

  • Nitrogen atomic mass = 14.01 g/mol
  • Hydrogen atomic mass = 1.01 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of HNO3 = 1.01 + 14.01 + 3(16.00) = 63.02 g/mol
  • Percentage composition of nitrogen = (14.01 g/mol / 63.02 g/mol) * 100% ≈ 22.22%

12. Question: Determine the percentage composition of carbon in acetic acid (CH3COOH).

Answer:

  • Carbon atomic mass = 12.01 g/mol
  • Hydrogen atomic mass = 1.01 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of CH3COOH = 2(12.01) + 4(1.01) + 2(16.00) = 60.06 g/mol
  • Percentage composition of carbon = (2 * 12.01 g/mol / 60.06 g/mol) * 100% ≈ 39.97%

13. Question: Calculate the percentage composition of sulfur in sulfur hexafluoride (SF6).

Answer:

  • Sulfur atomic mass = 32.07 g/mol
  • Fluorine atomic mass = 18.998 g/mol
  • Formula mass of SF6 = 32.07 + 6(18.998) = 146.05 g/mol
  • Percentage composition of sulfur = (32.07 g/mol / 146.05 g/mol) * 100% ≈ 21.98%

14. Question: Find the percentage composition of chlorine in perchloric acid (HClO4).

Answer:

  • Hydrogen atomic mass = 1.01 g/mol
  • Chlorine atomic mass = 35.45 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of HClO4 = 1.01 + 35.45 + 4(16.00) = 100.45 g/mol
  • Percentage composition of chlorine = (35.45 g/mol / 100.45 g/mol) * 100% ≈ 35.31%

15. Question: Determine the percentage composition of nitrogen in ammonium nitrate (NH4NO3).

Answer:

  • Nitrogen atomic mass = 14.01 g/mol
  • Hydrogen atomic mass = 1.01 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of NH4NO3 = 14.01 + 4(1.01) + 3(16.00) = 80.04 g/mol
  • Percentage composition of nitrogen = (14.01 g/mol / 80.04 g/mol) * 100% ≈ 17.50%

16. Question: Calculate the percentage composition of oxygen in hydrogen peroxide (H2O2).

Answer:

  • Hydrogen atomic mass = 1.01 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of H2O2 = 2(1.01) + 2(16.00) = 34.02 g/mol
  • Percentage composition of oxygen = (2 * 16.00 g/mol / 34.02 g/mol) * 100% ≈ 94.14%

17. Question: Find the percentage composition of sodium in sodium sulfate (Na2SO4).

Answer:

  • Sodium atomic mass = 22.99 g/mol
  • Sulfur atomic mass = 32.07 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of Na2SO4 = 2(22.99) + 32.07 + 4(16.00) = 142.06 g/mol
  • Percentage composition of sodium = (2 * 22.99 g/mol / 142.06 g/mol) * 100% ≈ 32.43%

18. Question: Determine the percentage composition of carbon in glucose (C6H12O6).

Answer:

  • Carbon atomic mass = 12.01 g/mol
  • Hydrogen atomic mass = 1.01 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of C6H12O6 = 6(12.01) + 12(1.01) + 6(16.00) = 180.18 g/mol
  • Percentage composition of carbon = (6 * 12.01 g/mol / 180.18 g/mol) * 100% ≈ 40.00%

19. Question: Calculate the percentage composition of phosphorus in phosphoric acid (H3PO4).

Answer:

  • Phosphorus atomic mass = 30.97 g/mol
  • Hydrogen atomic mass = 1.01 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of H3PO4 = 3(1.01) + 30.97 + 4(16.00) = 97.99 g/mol
  • Percentage composition of phosphorus = (30.97 g/mol / 97.99 g/mol) * 100% ≈ 31.60%

20. Question: Find the percentage composition of nitrogen in urea (CH4N2O).

Answer:

  • Nitrogen atomic mass = 14.01 g/mol
  • Hydrogen atomic mass = 1.01 g/mol
  • Carbon atomic mass = 12.01 g/mol
  • Oxygen atomic mass = 16.00 g/mol
  • Formula mass of CH4N2O = 12.01 + 4(1.01) + 2(14.01) + 16.00 = 60.06 g/mol
  • Percentage composition of nitrogen = (2 * 14.01 g/mol / 60.06 g/mol) * 100% ≈ 46.66%

These questions cover a variety of compounds and elements, and the calculations provide insights into how to determine the percentage composition based on the mole concept.


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