You find the molecular mass (more accurately called the formula mass) of an ion by summing the atomic masses of all the atoms present in the ion's chemical formula, just as you would for a neutral molecule.
Here's a breakdown of the process:
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Identify the Chemical Formula: Make sure you know the correct chemical formula of the ion, including the number of each type of atom and the overall charge. For example, sulfate is SO42- and ammonium is NH4+.
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Find the Atomic Masses: Look up the atomic mass of each element present in the ion on the periodic table. These values are typically given in atomic mass units (amu) or Daltons (Da). For example:
- Sulfur (S): ~32.06 amu
- Oxygen (O): ~16.00 amu
- Nitrogen (N): ~14.01 amu
- Hydrogen (H): ~1.01 amu
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Multiply and Sum: Multiply the atomic mass of each element by the number of times it appears in the chemical formula, and then add all the results together.
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Example 1: Sulfate ion (SO42-)
- (1 x Atomic mass of S) + (4 x Atomic mass of O) = Molecular mass of SO42-
- (1 x 32.06 amu) + (4 x 16.00 amu) = 96.06 amu
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Example 2: Ammonium ion (NH4+)
- (1 x Atomic mass of N) + (4 x Atomic mass of H) = Molecular mass of NH4+
- (1 x 14.01 amu) + (4 x 1.01 amu) = 18.05 amu
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Important Note on Charge: The charge of the ion (e.g., 2- or +) arises from a gain or loss of electrons. Electrons have a negligible mass compared to protons and neutrons (which contribute almost all of an atom's mass). Therefore, the charge of the ion does not significantly affect the calculated molecular mass. You don't need to add or subtract any mass based on the ion's charge.
In Summary: The process of determining the formula mass of an ion is identical to calculating the molecular mass of a neutral molecule. The presence of a charge is due to the gain or loss of electrons, which have a negligible impact on the overall mass.
