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How Do You Find the Mass of an Atom in a Compound?

Published in Chemistry Calculations 3 mins read

To find the mass of an atom within a compound, you primarily use the compound's molecular formula, the atomic masses of the elements, and molar mass calculations. Here's a breakdown of the process:

Steps to Determine the Mass of an Atom in a Compound

  1. Identify the Molecular Formula: Know the chemical formula of the compound (e.g., H₂O, NaCl).

  2. Find the Atomic Masses: Look up the atomic masses of each element in the compound from the periodic table. These are usually expressed in atomic mass units (amu).

  3. Calculate the Molar Mass of the Compound: The molar mass is the sum of the atomic masses of all the atoms in the compound's formula, expressed in grams per mole (g/mol).

    • Multiply the atomic mass of each element by the number of atoms of that element present in the compound.
    • Add these values together to get the molar mass.

    Example: For water (H₂O):

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

  4. Calculate the Mass Percentage of the Element: Determine the percentage of the element's mass compared to the total mass of the compound.

    • (Mass of element in one mole of the compound / Molar mass of the compound) * 100%

    Example: Mass percentage of Hydrogen in H₂O:

    • (2 1.01 g/mol) / 18.02 g/mol) 100% ≈ 11.21%

  5. Determine the Mass of the Element in a Given Sample: If you have a specific mass of the compound, you can calculate the mass of the element within that sample.

    • Mass of element = (Mass percentage of element / 100) * Mass of compound sample

    Example: If you have 50 grams of H₂O, the mass of hydrogen is:

    • (11.21/100) * 50 g ≈ 5.61 grams

Example: Finding the Mass of Sodium in Sodium Chloride (NaCl)

  1. Molecular Formula: NaCl

  2. Atomic Masses:

    • Sodium (Na) ≈ 22.99 amu
    • Chlorine (Cl) ≈ 35.45 amu

  3. Molar Mass of NaCl: 22.99 + 35.45 = 58.44 g/mol

  4. Mass Percentage of Sodium in NaCl: (22.99 g/mol / 58.44 g/mol) * 100% ≈ 39.34%

  5. Mass of Sodium in 100g of NaCl: (39.34 / 100) * 100g = 39.34 grams

Summary

Finding the mass of an element within a compound involves using the molecular formula, atomic masses, and molar mass calculations to determine the percentage of the element by mass, which can then be used to find the mass of the element in any given sample of the compound.

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