Answer
Empirical Formula: This gives the simplest ratio of the elements that make up a compound.
Molecular Formula: This gives the number of each element that exists in a compound.
Structural Formula: This shows how the elements that make up the compound are arranged in space.
Work Step by Step
The empirical formula of an arbitrary compound denotes the relative number of atoms of element A there are compared to element B. For example, the empirical formula for the organic compound ethane is CH$_3$ because each molecule of ethane contains three atoms of hydrogen per atom of carbon.
The molecular formula of an arbitrary compound denotes the specific number of atoms of element A, B, etc. there are. For example, the molecular formula for ethane is C$_2$H$_6$ because each molecule of ethane contains two carbon atoms and six hydrogen atoms.
Lastly, the structural formula for compounds show you how the atoms in that given compound are bonded to one another (arranged). For example, the structural formula for ethane is CH$_3$CH$_3$ because each molecule of ethane contains a carbon bonded to three hydrogen forming the left side and a carbon bonded to three hydrogen forming the right side.
