How to Find the Empirical and Molecular Formula of a Compound
Chemical formulas represent the types and number of atoms in a compound. There are two types of formulas: empirical and molecular. The empirical formula shows the simplest whole number ratio of atoms in a compound, while the molecular formula shows the actual number of atoms in a molecule. Knowing how to find these formulas is essential for chemists to understand the properties and behavior of different compounds.
To find the empirical formula of a compound, you need to know the mass of each element in the compound and the overall mass of the compound. The empirical formula represents the simplest whole-number ratio of atoms in a compound, so the first step is to determine the number of moles of each element in the compound. To do this, you need to divide the mass of each element by its molar mass, which can be found on the periodic table.
Once you have the number of moles for each element, you need to determine the smallest mole ratio. Divide each mole value by the smallest mole value obtained in step 2. If the ratio obtained is not a whole number, multiply the values by a factor to get the nearest whole-number ratio. The empirical formula can then be written using the whole number mole ratios.
Finding the Empirical Formula of a Compound Example:
Suppose you have a compound with 36.5% sodium, 25.4% sulfur, and 38.1% oxygen. To find the empirical formula, you would first determine the number of moles of each element in the compound. Sodium has a molar mass of 22.99 g/mol, sulfur has a molar mass of 32.06 g/mol, and oxygen has a molar mass of 16.00 g/mol. Dividing the percentage of each element by its molar mass gives you the number of moles of each element.
Sodium: 36.5 g / 22.99 g/mol = 1.59 mol
Sulfur: 25.4 g / 32.06 g/mol = 0.79 mol
Oxygen: 38.1 g / 16.00 g/mol = 2.38 mol
The smallest mole ratio is 0.79 mol of sulfur, which means you need to divide each mole value by 0.79. Dividing the moles by 0.79 gives you the whole number mole ratios:
Sodium: 2.01
Sulfur: 1
Oxygen: 3
The empirical formula is Na2SO3, which represents the simplest whole-number ratio of atoms in the compound.
To determine the molecular formula, you need to know the molecular mass of the compound, which is the sum of the masses of all atoms in the molecule. You also need to know the empirical formula. Then, you can calculate the empirical formula mass by adding the atomic masses of the elements in the empirical formula. The molecular formula mass is the actual molecular weight of the compound, which can be determined experimentally.
To find the whole number multiple of the empirical formula, divide the molecular mass of the compound by the empirical formula mass. The whole number multiple is the number of empirical formula units present in the molecule. The molecular formula can be obtained by multiplying the empirical formula by the whole number multiple.
For example, suppose the empirical formula of a compound is CH2O, and the molecular weight is 180 g/mol. The empirical formula mass is 30 g/mol, which is the sum of the atomic masses of carbon, hydrogen, and oxygen. Dividing the molecular weight by the empirical formula mass gives you the whole number multiple:
Whole number multiple = 180 g/mol ÷ 30 g/mol = 6
The molecular formula is then obtained: C6H12O6
