Relative formula mass for ionic compounds is calculated in the same way as for molecular compounds, even though ionic compounds don't exist as discrete molecules. The calculation utilizes the compound's empirical formula.
Understanding Relative Formula Mass
The relative formula mass (often abbreviated as Mr) is the sum of the relative atomic masses (Ar) of all the atoms in a formula unit of a substance. While we often associate "formula mass" with ionic compounds (since they don't form molecules), the method of calculation is identical to finding the relative molecular mass of a covalent compound. The key is understanding what the "formula" represents in each case.
Ionic Compounds and Empirical Formulas
Ionic compounds, like sodium chloride (NaCl), exist as a giant lattice structure of ions, not as individual molecules. Therefore, we use their empirical formula to determine the relative formula mass. The empirical formula represents the simplest whole-number ratio of the ions in the compound.
Calculation Example: Sodium Chloride (NaCl)
Let's calculate the relative formula mass of sodium chloride (NaCl):
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Identify the elements and their symbols: Sodium (Na) and Chlorine (Cl).
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Find the relative atomic masses (Ar) from the periodic table:
- Ar(Na) = 23
- Ar(Cl) = 35.5
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Use the empirical formula (NaCl) to determine the number of atoms of each element: One sodium atom and one chlorine atom.
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Calculate the relative formula mass (Mr):
Mr(NaCl) = Ar(Na) + Ar(Cl) = 23 + 35.5 = 58.5
Therefore, the relative formula mass of sodium chloride is 58.5.
Why Use Empirical Formulas?
Since ionic compounds don't form molecules, the empirical formula provides the simplest representation of the ratio of ions in the compound. It accurately reflects the composition and is used consistently for calculating the relative formula mass. We cannot talk of a single "molecule" of NaCl, but the ratio of Na to Cl is 1:1 throughout the giant ionic lattice.
Key Takeaways
- The relative formula mass of an ionic compound is calculated using its empirical formula.
- The method of calculation is the same as for relative molecular mass of covalent compounds.
- The empirical formula represents the simplest whole-number ratio of ions in the compound.
- This calculation provides a useful measure for stoichiometry and quantitative analysis, just as relative molecular mass does for molecular compounds.
