Empirical formula vs. molecular formula explained with examples

📚 Quick Study Guide

• 🧪 The empirical formula represents the simplest whole number ratio of atoms in a compound.
• ➗ To find the empirical formula, you need the percent composition or mass data, then convert to moles, and finally find the simplest whole number ratio.
• ⚛️ The molecular formula represents the actual number of atoms of each element in a molecule.
• ⚖️ The molecular formula is a multiple of the empirical formula: Molecular Formula = (Empirical Formula)$_n$, where $n$ is a whole number.
• 🔢 To find the molecular formula, you need the empirical formula and the molar mass of the compound. Calculate $n$ using the formula: $n = \frac{\text{Molar Mass}}{\text{Empirical Formula Mass}}$.
• 💡 Remember that some compounds can have the same empirical and molecular formulas (e.g., $H_2O$).

🧪 Practice Quiz

1. What does the empirical formula represent?
1. The actual number of atoms in a molecule.
2. The simplest whole number ratio of atoms in a compound.
3. The molar mass of a compound.
4. The percent composition of a compound.
2. A compound has an empirical formula of $CH_2O$ and a molar mass of 180 g/mol. What is its molecular formula?
1. $C_3H_6O_3$
2. $C_6H_{12}O_6$
3. $CH_2O$
4. $C_9H_{18}O_9$
3. Which of the following compounds has the same empirical and molecular formula?
1. $H_2O$
2. $C_6H_{12}O_6$
3. $C_2H_4$
4. $C_4H_{10}$
4. What information is needed to determine the molecular formula of a compound?
1. The empirical formula only.
2. The molar mass only.
3. The empirical formula and the molar mass.
4. The percent composition only.
5. The empirical formula of a compound is $NO_2$. Its molar mass is 92.02 g/mol. What is its molecular formula?
1. $NO_2$
2. $N_2O_4$
3. $N_3O_6$
4. $N_4O_8$
6. A compound is found to contain 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is its empirical formula?
1. $CHO$
2. $CH_2O$
3. $C_2HO$
4. $C_2H_2O$
7. What is the value of 'n' if the empirical formula is $CH_3$ and the molecular formula is $C_2H_6$?
1. 1
2. 2
3. 3
4. 4