Chemical formulae
Chemical formulae
- A formula shows which atoms are in a substance and how many of each.
- The molecular formula is the actual number of each atom — glucose is $\text{C}_6\text{H}_{12}\text{O}_6$.
- The empirical formula is the simplest whole-number ratio — glucose is $\text{CH}_2\text{O}$.
Practice
The empirical formula of glucose (C6H12O6) is:
Divide each subscript by the highest common factor (6): C6H12O6 → CH2O.
Practice
The empirical formula is:
The empirical formula is the simplest ratio; the molecular formula is the actual count.
Formulae from ion charges
- For an ionic compound, the total $+$ charge must balance the total $-$ charge (no overall charge).
- Example: $\text{Na}^{+}$ and $\text{O}^{2-}$ → you need two $\text{Na}^{+}$ per $\text{O}^{2-}$, so the formula is $\text{Na}_2\text{O}$.
Practice
What is the formula of the compound formed by Na+ and O2-?
Two Na+ (total +2) balance one O2- (−2), giving Na2O.
You've got it
Key idea
- molecular formula = actual atom count ($\text{C}_6\text{H}_{12}\text{O}_6$); empirical = simplest ratio ($\text{CH}_2\text{O}$)
- in an ionic compound the charges must balance to zero
- $\text{Na}^{+}$ + $\text{O}^{2-}$ → $\text{Na}_2\text{O}$