Calculating the molecular mass of a compound is straightforward. You simply add up the atomic masses of all the atoms present in the molecule.
Step-by-Step Guide
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Identify the Molecular Formula: Determine the precise chemical formula of the compound. This formula shows the type and number of each atom in the molecule (e.g., H₂O, CO₂, C₆H₁₂O₆).
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Find Atomic Masses: Use a periodic table like this one to find the atomic mass (in atomic mass units, or amu) of each element in the formula. Remember that atomic mass is typically a weighted average of the isotopes of an element.
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Multiply and Sum: Multiply the atomic mass of each element by the number of times it appears in the molecular formula. Then, add up all these products to get the total molecular mass.
Examples
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Water (H₂O):
- Hydrogen (H) atomic mass ≈ 1.01 amu; there are 2 hydrogen atoms.
- Oxygen (O) atomic mass ≈ 16.00 amu; there is 1 oxygen atom.
- Molecular mass = (2 × 1.01 amu) + (1 × 16.00 amu) = 18.02 amu
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Carbon Dioxide (CO₂):
- Carbon (C) atomic mass ≈ 12.01 amu; there is 1 carbon atom.
- Oxygen (O) atomic mass ≈ 16.00 amu; there are 2 oxygen atoms.
- Molecular mass = (1 × 12.01 amu) + (2 × 16.00 amu) = 44.01 amu
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Glucose (C₆H₁₂O₆):
- Carbon (C) atomic mass ≈ 12.01 amu; there are 6 carbon atoms.
- Hydrogen (H) atomic mass ≈ 1.01 amu; there are 12 hydrogen atoms.
- Oxygen (O) atomic mass ≈ 16.00 amu; there are 6 oxygen atoms.
- Molecular mass = (6 × 12.01 amu) + (12 × 1.01 amu) + (6 × 16.00 amu) = 180.18 amu
Remember that the term molecular mass is often used interchangeably with molar mass, which represents the mass of one mole (6.022 x 10²³) of molecules and is typically expressed in grams per mole (g/mol). The numerical value is the same, only the units differ. The provided references confirm this method and the use of the periodic table to obtain atomic masses.
