Understanding Empirical and Molecular Formulas
In chemistry, empirical and molecular formulas are two types of chemical formulas used to represent the composition of a molecule. While they are related, they serve different purposes and provide different information about the molecule. In this worksheet, we will explore the concepts of empirical and molecular formulas, and provide solutions to help you better understand these concepts.
Empirical Formulas
An empirical formula represents the simplest whole-number ratio of atoms of each element present in a molecule. It is also known as the simplest formula. To determine the empirical formula, we need to know the mass percentage of each element in the compound.
For example, consider a compound that contains 40% carbon, 6.67% hydrogen, and 53.33% oxygen by mass. To find the empirical formula, we can start by assuming we have 100 grams of the compound.
📝 Note: When working with percentages, it is often helpful to assume a 100-gram sample of the compound.
| Element | Mass Percentage | Number of Moles |
|---|---|---|
| Carbon | 40% | 40 g / 12.01 g/mol = 3.33 mol |
| Hydrogen | 6.67% | 6.67 g / 1.008 g/mol = 6.62 mol |
| Oxygen | 53.33% | 53.33 g / 16.00 g/mol = 3.33 mol |
Now, we divide each number of moles by the smallest number of moles to get the simplest ratio.
| Element | Simplest Ratio |
|---|---|
| Carbon | 3.33 mol / 3.33 mol = 1 |
| Hydrogen | 6.62 mol / 3.33 mol = 2 |
| Oxygen | 3.33 mol / 3.33 mol = 1 |
The empirical formula is CH2O.
Molecular Formulas
A molecular formula represents the actual number of atoms of each element present in a molecule. To determine the molecular formula, we need to know the empirical formula and the molar mass of the compound.
For example, consider a compound with the empirical formula CH2O and a molar mass of 60.05 g/mol.
To find the molecular formula, we can start by calculating the molar mass of the empirical formula.
| Element | Number of Moles | Molar Mass |
|---|---|---|
| Carbon | 1 | 12.01 g/mol |
| Hydrogen | 2 | 2.016 g/mol |
| Oxygen | 1 | 16.00 g/mol |
The molar mass of the empirical formula is 30.03 g/mol.
Since the molar mass of the compound is 60.05 g/mol, we can divide by the molar mass of the empirical formula to get the multiplier.
60.05 g/mol / 30.03 g/mol = 2
The molecular formula is twice the empirical formula, so it is C2H4O2.
Worksheet Solutions
- A compound contains 30% carbon, 4.29% hydrogen, and 65.71% oxygen by mass. What is the empirical formula?
| Element | Mass Percentage | Number of Moles |
|---|---|---|
| Carbon | 30% | 30 g / 12.01 g/mol = 2.5 mol |
| Hydrogen | 4.29% | 4.29 g / 1.008 g/mol = 4.26 mol |
| Oxygen | 65.71% | 65.71 g / 16.00 g/mol = 4.11 mol |
| Element | Simplest Ratio |
|---|---|
| Carbon | 2.5 mol / 2.11 mol = 1.18 ≈ 1 |
| Hydrogen | 4.26 mol / 2.11 mol = 2.02 ≈ 2 |
| Oxygen | 4.11 mol / 2.11 mol = 1.95 ≈ 2 |
The empirical formula is CH2O2.
- A compound has the empirical formula CH2O and a molar mass of 90.08 g/mol. What is the molecular formula?
| Element | Number of Moles | Molar Mass |
|---|---|---|
| Carbon | 1 | 12.01 g/mol |
| Hydrogen | 2 | 2.016 g/mol |
| Oxygen | 1 | 16.00 g/mol |
The molar mass of the empirical formula is 30.03 g/mol.
Since the molar mass of the compound is 90.08 g/mol, we can divide by the molar mass of the empirical formula to get the multiplier.
90.08 g/mol / 30.03 g/mol = 3
The molecular formula is three times the empirical formula, so it is C3H6O3.
- A compound contains 40% carbon, 6.67% hydrogen, and 53.33% oxygen by mass. What is the empirical formula?
| Element | Mass Percentage | Number of Moles |
|---|---|---|
| Carbon | 40% | 40 g / 12.01 g/mol = 3.33 mol |
| Hydrogen | 6.67% | 6.67 g / 1.008 g/mol = 6.62 mol |
| Oxygen | 53.33% | 53.33 g / 16.00 g/mol = 3.33 mol |
| Element | Simplest Ratio |
|---|---|
| Carbon | 3.33 mol / 3.33 mol = 1 |
| Hydrogen | 6.62 mol / 3.33 mol = 2 |
| Oxygen | 3.33 mol / 3.33 mol = 1 |
The empirical formula is CH2O.
What is the difference between an empirical formula and a molecular formula?
+
An empirical formula represents the simplest whole-number ratio of atoms of each element present in a molecule, while a molecular formula represents the actual number of atoms of each element present in a molecule.
How do you determine the empirical formula of a compound?
+
To determine the empirical formula, you need to know the mass percentage of each element in the compound. You can then calculate the number of moles of each element and divide by the smallest number of moles to get the simplest ratio.
How do you determine the molecular formula of a compound?
+
To determine the molecular formula, you need to know the empirical formula and the molar mass of the compound. You can then calculate the molar mass of the empirical formula and divide the molar mass of the compound by the molar mass of the empirical formula to get the multiplier.
