To find the formula of an unknown compound, you'll typically determine its empirical formula first, then use that to find the molecular formula if the molar mass is known.
Steps to Determine the Formula
1. Determine the Empirical Formula
The empirical formula shows the simplest whole number ratio of atoms in a compound. This involves:
- Finding the mass or percentage of each element: Experimental data provides these values.
- Converting to moles: Divide the mass of each element by its atomic mass to get the number of moles.
- Finding the mole ratio: Divide the number of moles of each element by the smallest number of moles calculated. This provides the simplest ratio.
- Adjusting to whole numbers: If the ratio is not in whole numbers, multiply all the numbers by a common factor to obtain whole numbers. These whole numbers become the subscripts in the empirical formula.
2. Determine the Molecular Formula
The molecular formula shows the actual number of atoms of each element in a molecule. To determine this:
- Calculate the empirical formula mass: Add the atomic masses of all atoms in the empirical formula.
- Determine the ratio between molar mass and empirical formula mass: Divide the molar mass of the compound by the empirical formula mass. As stated in the reference, "The result should be a whole number or very close to a whole number."
- Multiply subscripts in the empirical formula: Multiply all the subscripts in the empirical formula by the whole number found in step 2. The result is the molecular formula.
Example:
Let’s imagine a compound where you find the empirical formula to be CH2O and you know its molar mass is approximately 180 g/mol.
| Steps | Explanation | Calculation | Result |
|---|---|---|---|
| 1. Calculate empirical formula mass | Sum of the atomic masses from the empirical formula CH2O (12.01 + 2(1.01) + 16.00) | 12.01 + 2.02 + 16.00 | 30.03 g/mol |
| 2. Determine molar mass ratio | Molar mass / empirical formula mass | 180 g/mol / 30.03 g/mol | Approximately 6 |
| 3. Determine Molecular Formula | Multiply the subscripts in CH2O by 6 | C1x6H2x6O1x6 | C6H12O6 |
Therefore, the molecular formula for this compound is C6H12O6.
Key Insights
- The empirical formula is the simplified ratio, while the molecular formula is the actual ratio.
- If the ratio of molar mass to empirical formula mass is 1, the empirical formula is also the molecular formula.
- Experimental methods, such as combustion analysis or mass spectrometry, are used to acquire data necessary for calculations.
