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What is mass fraction of an element in a compound?

Published in Chemical Composition 2 mins read

The mass fraction of an element in a compound represents the proportion of that element's mass compared to the total mass of the compound.

Understanding Mass Fraction

The mass fraction is a way to express the composition of a chemical compound. It answers the question: "Out of the total mass of this compound, what fraction is attributed to this specific element?". It's always expressed as a value between 0 and 1 (or as a percentage).

How to Calculate Mass Fraction

According to the reference, the mass fraction of an element in a compound is calculated by:

  • Comparing the mass of the particular element present in one mole of the compound to the total mass of the compound.

Here's a more detailed breakdown:

  1. Find the molar mass of the compound: This is done by adding up the atomic masses of all the atoms in the compound's formula.

  2. Find the molar mass of the element: Use the atomic mass of the element multiplied by the number of atoms of that element in the compound.

  3. Divide the mass of the element by the total mass of the compound:

    Mass Fraction of Element = (Mass of element in one mole of compound) / (Molar mass of the compound)

Example

Let's calculate the mass fraction of hydrogen in water (H2O).

  1. Molar mass of water (H2O):
    • Hydrogen (H): 1.008 g/mol * 2 = 2.016 g/mol
    • Oxygen (O): 16.00 g/mol * 1 = 16.00 g/mol
    • Total molar mass of H2O = 2.016 + 16.00 = 18.016 g/mol
  2. Mass of hydrogen in one mole of water: 2.016 g/mol
  3. Mass Fraction: (2.016 g/mol) / (18.016 g/mol) = 0.1119 (approximately)

The mass fraction of hydrogen in water is approximately 0.1119, or about 11.19%. This means that hydrogen contributes to around 11.19% of the total mass of water.

Importance of Mass Fraction

  • Chemical Analysis: It helps to determine the purity of a substance by calculating the percentage of a particular element present.
  • Stoichiometry: Used in chemical reactions to understand how much of each element is involved in a particular process.
  • Material Science: Important in the analysis of materials and in determining their composition.

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