Formula Mass
Calculates the sum of the atomic masses of all atoms in a formula unit of an ionic compound, used for substances that do not exist as discrete molecules.
What is Formula Mass?
The simplest whole-number ratio of ions in an ionic compound or atoms in a network solid, representing the composition of the substance.
Key formula / rule: Formula Mass Calculation
Key points
- Define formula mass.
- Differentiate between molecular mass and formula mass.
- Calculate the formula mass of ionic compounds and network solids.
- Relate formula mass to the concept of the formula unit.
Common exam trap
Confusing formula mass with molecular mass.
Definitions
- Term
Formula Unit
- Meaning
The simplest whole-number ratio of ions in an ionic compound or atoms in a network solid, representing the composition of the substance.
- Term
Formula Mass
- Meaning
The sum of the atomic masses of all atoms in one formula unit of a compound.
Learning objectives
Define formula mass.
Differentiate between molecular mass and formula mass.
Calculate the formula mass of ionic compounds and network solids.
Relate formula mass to the concept of the formula unit.
Formulae
- Name
Formula Mass Calculation
- Note
This applies to ionic compounds and network solids.
- Expression
Formula Mass = Σ (Atomic Mass of Element × Number of atoms of that Element in the formula unit)
Prerequisites
Understanding of atomic mass.
Knowledge of chemical formulas and subscripts.
Familiarity with the periodic table.
Common mistakes
Confusing formula mass with molecular mass.
Incorrectly identifying the number of atoms of each element in the formula unit.
Using incorrect atomic masses from the periodic table.
Forgetting to multiply the atomic mass by the correct subscript for each element.
Keywords
Formula mass
Ionic compound
Formula unit
Atomic mass
Stoichiometry
Molar mass
Network solid
Practice preview
The formula mass of hydrated copper(II) sulfate, CuSO₄·5H₂O, is approximately 249.5 u. If the atomic mass of copper (Cu) is 63.5 u, what is the formula mass of anhydrous copper(II) sulfate (CuSO₄)? (Atomic masses: S = 32…
medium
What is the formula mass of sodium chloride (NaCl)? (Atomic masses: Na = 23 u, Cl = 35.5 u)…
easy
Consider the compound aluminum sulfate, Al₂(SO₄)₃. Which statement is INCORRECT regarding its formula mass calculation? (Atomic masses: Al = 27 u, S = 32 u, O = 16 u)…
medium