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Elemental Composition of Pure Substances

Introduction​

Every pure substance, whether it is an element or a compound, has a definite and fixed composition. The elemental composition of a pure substance refers to the percentage of each element present in that substance by mass. This concept is central to chemical analysis, allowing chemists to determine the formula of compounds, compare experimental results with theoretical predictions, and verify the purity of samples.

Pure Substances​

A pure substance is matter that has a uniform and definite composition. Pure substances can be classified as:

  • Elements: Substances made of only one type of atom. Examples: oxygen (O₂), gold (Au), carbon (C).
  • Compounds: Substances made of two or more elements chemically bonded in fixed proportions. Examples: water (H₂O), sodium chloride (NaCl), glucose (C₆H₁₂O₆).

Determining Elemental Composition​

The elemental composition of a compound can be calculated using its chemical formula and the atomic masses of its constituent elements.

Formula:
Percent by Mass of Element = (Mass of Element in 1 Mole of Compound / Molar Mass of Compound) × 100

Example 1: Water (H₂O)​

  • Molar mass of H₂O = (2 × 1.01 g/mol) + (16.00 g/mol) = 18.02 g/mol
  • Mass of H = 2.02 g/mol
  • Mass of O = 16.00 g/mol

Percent by mass of H = (2.02 / 18.02) × 100 ≈ 11.2%
Percent by mass of O = (16.00 / 18.02) × 100 ≈ 88.8%

Example 2: Carbon Dioxide (CO₂)​

  • Molar mass of CO₂ = (12.01 g/mol) + (2 × 16.00 g/mol) = 44.01 g/mol
  • Mass of C = 12.01 g/mol
  • Mass of O = 32.00 g/mol

Percent by mass of C = (12.01 / 44.01) × 100 ≈ 27.3%
Percent by mass of O = (32.00 / 44.01) × 100 ≈ 72.7%

Empirical and Molecular Formulas​

Elemental composition can also help determine a compound’s formula:

  • Empirical formula: The simplest whole-number ratio of atoms in a compound.
  • Molecular formula: The actual number of atoms of each element in one molecule.

For example, glucose has a molecular formula of C₆H₁₂O₆ but an empirical formula of CH₂O.

Importance of Elemental Composition​

  • Verification of Purity: Ensures that a substance is pure and not contaminated.
  • Chemical Identification: Helps confirm the identity of an unknown compound.
  • Stoichiometry: Provides the basis for balanced chemical equations and quantitative chemical calculations.
  • Industrial Applications: Used in pharmaceuticals, materials science, and food chemistry to ensure correct formulations.

Conclusion​

The elemental composition of pure substances provides essential information about their chemical makeup. By calculating the percent by mass of each element, chemists can determine empirical and molecular formulas, verify experimental data, and understand chemical properties. This concept is fundamental to both theoretical and applied chemistry.

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