To write the molecular formula of a compound, you need to know the chemical symbols of all the elements present in the compound and the number of atoms of each element in a single molecule.
Elements and Their Quantities
A molecular formula precisely represents the number and types of atoms in a single molecule of a compound. This differs from an empirical formula, which only represents the simplest whole-number ratio of atoms.
Here's a breakdown of the required information:
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Chemical Symbols: You must know the correct chemical symbol for each element present in the compound (e.g., H for hydrogen, O for oxygen, C for carbon).
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Number of Atoms (Subscripts): You need to determine how many atoms of each element are present in one molecule of the compound. This is indicated by a subscript written to the right of the element's symbol. If there is only one atom of an element, the subscript '1' is typically omitted.
Example
Consider the molecular formula for glucose, which is C6H12O6.
- Elements Present: Carbon (C), Hydrogen (H), and Oxygen (O)
- Number of Atoms:
- Carbon: 6 atoms
- Hydrogen: 12 atoms
- Oxygen: 6 atoms
This molecular formula tells us that each molecule of glucose contains exactly 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. The empirical formula of glucose would be CH2O, showing the simplest whole number ratio of the atoms.
Distinguishing Molecular and Empirical Formulas
It is important to note the distinction between molecular and empirical formulas. The molecular formula gives the exact number of each type of atom in a molecule, whereas the empirical formula gives the simplest whole-number ratio of atoms in a compound. For example, hydrogen peroxide has the molecular formula H2O2, but its empirical formula is HO.
In summary, to write a correct molecular formula, you must identify each element present and know the exact number of each element's atoms contained within a single molecule of the substance.
