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3.10: Molecular Weight

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    430560

    • Deboleena Roy (American River College)
    • LibreTexts
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     Learning Objectives
    • Determine the molecular weight of a molecule.

    Ions in ionic compounds, are arranged in lattices called crystals, molecules of covalent compounds exist as discrete units with a characteristic mass and a certain three-dimensional shape.

    Molecular Weight

    The mass of a molecule—the molecular weight (also called the molecular mass)—is simply the sum of the atomic weights of its atoms. As with formula weights, it is important that you keep track of the number of atoms of each element in the molecular formula to obtain the correct molecular weight.

    Example \(\PageIndex{1}\)

    What is the molecular weight of each covalent compound?

    1. H2O
    2. C6H6
    3. NO2
    4. N2O5
    Solution

    Use the atomic weights from the Periodic Table.

    The molecular formula H2O indicates that there are two hydrogen atoms and one oxygen atom in each molecule. Summing the atomic weights of these atoms,

    This periodic table finds the atomic weight of atoms.
    2 H: 2 × 1.01 = 2.02 amu
    1 O:   + 16.00 amu
    Total:   18.02 amu

    The molecular weight of H2O is 18.02 amu.

    This periodic table finds the atomic weight of atoms.
    6 C: 6 × 12.01 = 72.06 amu
    6 H: 6 × 1.01 = + 6.06 amu
    Total:   78.12 amu

    The molecular weight of C6H6 is 78.12 amu.

    This periodic table finds the atomic weight of atoms.
    1 N:   14.01 amu
    2 O: 2 × 16.00 = + 32.00 amu
    Total:   46.01 amu

    The molecular weight of NO2 is 46.01 amu.

    This periodic table finds the atomic weight of atoms.
    2 N: 2 × 14.01 = 28.02 amu
    5 O: 5 × 16.00 = + 80.00 amu
    Total:   108.02 amu

    The molecular weight of N2O5 is 108.02 amu.

    Note that the two different nitrogen and oxygen compounds in these examples have different molecular masses.

    Exercise \(\PageIndex{1}\)

    What is the molecular weight of each covalent compound?

    1. C2H2
    2. CO
    3. CO2
    4. BF3
    Answer a:

    26.04 amu

    Answer b:

    28.01 amu

    Answer c:

    44.01 amu

    Answer d:

    67.81 amu

    Key Takeaways

    • A molecule has a certain mass, called the molecular weight.

    This page titled 3.10: Molecular Weight is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Deboleena Roy (American River College) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.