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How to Calculate Weight Percent of an Element in a Compound?

Published in Chemical Composition 3 mins read

To calculate the weight percent (also known as mass percent) of an element within a compound, you need to determine what portion of the compound's total mass is made up by that specific element. This is achieved by comparing the mass of the element in one mole of the compound to the molar mass of the entire compound.

Calculating Weight Percent: Step-by-Step

Here's a detailed breakdown of the calculation process, as outlined in the provided reference:

  1. Determine the Molar Mass of the Compound:

    • First, find the atomic mass of each element in the compound from the periodic table.
    • Multiply each atomic mass by the number of atoms of that element present in the chemical formula.
    • Add all these values to find the molar mass of the compound.

  2. Determine the Mass of the Element of Interest:

    • Find the atomic mass of the element of interest.
    • Multiply the element’s atomic mass by the number of atoms of that element present in one mole of the compound.

  3. Calculate the Weight Percent:

    • Divide the total mass of the element of interest (from Step 2) by the molar mass of the compound (from Step 1).
    • Multiply the result by 100% to express the answer as a percentage.

    The reference states the formula as:

    Weight Percent of Element = (Mass of element in 1 mole of compound / Molar mass of compound) x 100%

Example

Let's calculate the weight percent of hydrogen in water (H2O):

  • Step 1: Find the Molar Mass of Water (H2O)
    • Atomic mass of Hydrogen (H) = 1.008 g/mol
    • Atomic mass of Oxygen (O) = 16.00 g/mol
    • Molar mass of H2O = (2 x 1.008 g/mol) + 16.00 g/mol = 18.016 g/mol
  • Step 2: Determine the Mass of Hydrogen in 1 mole of water
    • There are 2 hydrogen atoms in one molecule of water.
    • Mass of Hydrogen = 2 x 1.008 g/mol = 2.016 g/mol
  • Step 3: Calculate the Weight Percent of Hydrogen
    • Weight % of Hydrogen = (2.016 g/mol / 18.016 g/mol) x 100% = 11.19%

Therefore, hydrogen makes up approximately 11.19% of the total mass of water.

Important Insights

  • The weight percentages of all elements in a compound should always add up to 100%, as the reference also points out.
  • This calculation is essential in various chemical applications, such as stoichiometry, analytical chemistry, and materials science, where understanding the composition of substances is crucial.

By following these steps, you can accurately determine the weight percent of any element within a given compound.

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