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How to Calculate the Mass of an Element in a Compound?

Published in Chemical Composition 3 mins read

To calculate the mass of an element in a compound, you need to determine the element's mass contribution to the compound's overall molar mass. This involves calculating the mass percent of the element in the compound and then applying that percentage to a given mass of the compound.

Here's a step-by-step breakdown:

Steps to Calculate the Mass of an Element in a Compound:

  1. Determine the Chemical Formula of the Compound: You need to know the exact chemical formula (e.g., H₂O, NaCl, CO₂) to identify the elements present and their respective quantities.

  2. Calculate the Molar Mass of the Compound:

    • Find the atomic mass of each element in the compound from the periodic table.
    • Multiply the atomic mass of each element by the number of atoms of that element present in the formula.
    • Add up the results for all elements in the compound. This gives you the molar mass of the compound in grams per mole (g/mol).

  3. Calculate the Mass of the Element in One Mole of the Compound:

    • Multiply the atomic mass of the element by the number of atoms of that element present in the compound's formula. This result represents the total mass of that element present in one mole of the compound.

  4. Calculate the Mass Percent of the Element in the Compound:

    • Divide the mass of the element in one mole of the compound (calculated in step 3) by the molar mass of the entire compound (calculated in step 2).
    • Multiply the result by 100 to express it as a percentage.

    Formula:

    Mass Percent of Element = (Mass of Element in 1 mole of Compound / Molar Mass of Compound) * 100%

  5. Calculate the Mass of the Element in a Given Sample of the Compound:

    • Multiply the mass percent (expressed as a decimal; divide the mass percent by 100) of the element by the total mass of the compound sample.

    Formula:

    Mass of Element in Sample = (Mass Percent of Element / 100) * Total Mass of Compound Sample

Example: Calculating the Mass of Hydrogen in 50g of Water (H₂O)

  1. Chemical Formula: H₂O

  2. Molar Mass of H₂O:

    • Atomic mass of H (Hydrogen): 1.01 g/mol
    • Atomic mass of O (Oxygen): 16.00 g/mol
    • Molar mass of H₂O = (2 1.01 g/mol) + (1 16.00 g/mol) = 18.02 g/mol

  3. Mass of Hydrogen in 1 mole of H₂O:

    • (2 * 1.01 g/mol) = 2.02 g/mol

  4. Mass Percent of Hydrogen in H₂O:

    • (2.02 g/mol / 18.02 g/mol) * 100% = 11.21%

  5. Mass of Hydrogen in 50g of H₂O:

    • (11.21 / 100) * 50g = 5.605g

Therefore, there are approximately 5.605 grams of hydrogen in 50 grams of water.

Summary

Calculating the mass of an element within a compound involves determining the mass percent of that element and applying it to a given mass of the compound. This process requires knowledge of the compound's chemical formula, molar mass calculations, and basic percentage calculations.

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