Steps:
1. Assume a 100 g sample: This makes the percentages directly translate to grams. For example, if a compound is 50% carbon, you'll assume 50 grams of carbon in your 100 gram sample.
2. Convert grams to moles: Divide the mass of each element by its molar mass (found on the periodic table). This will give you the moles of each element in the sample.
3. Find the simplest whole-number ratio of moles: Divide each of the mole values you calculated by the smallest mole value. This will give you the mole ratio of the elements in the compound.
4. The empirical formula: The resulting ratio from step 3 represents the empirical formula, which is the simplest whole-number ratio of atoms in the compound.
5. Determine the molecular formula:
* If you know the molar mass of the compound: Divide the molecular mass by the empirical formula mass. This will give you the whole-number factor by which to multiply the subscripts in the empirical formula to get the molecular formula.
* If you don't know the molar mass: You can only determine the empirical formula.
Example:
Let's say a compound is found to have the following percentage composition:
* Carbon: 40.0%
* Hydrogen: 6.7%
* Oxygen: 53.3%
Here's how to find its molecular formula:
1. Assume 100 g sample:
* Carbon: 40.0 g
* Hydrogen: 6.7 g
* Oxygen: 53.3 g
2. Convert to moles:
* Carbon: 40.0 g / 12.01 g/mol = 3.33 mol
* Hydrogen: 6.7 g / 1.01 g/mol = 6.63 mol
* Oxygen: 53.3 g / 16.00 g/mol = 3.33 mol
3. Find simplest ratio:
* Carbon: 3.33 mol / 3.33 mol = 1
* Hydrogen: 6.63 mol / 3.33 mol = 2
* Oxygen: 3.33 mol / 3.33 mol = 1
4. Empirical formula: CH₂O
5. Molecular formula: Let's say the molar mass of the compound is 180 g/mol.
* Empirical formula mass: 12.01 + (2 * 1.01) + 16.00 = 30.03 g/mol
* Factor: 180 g/mol / 30.03 g/mol = 6
* Molecular formula: (CH₂O) * 6 = C₆H₁₂O₆
Important Notes:
* Make sure the percentage composition adds up to 100%.
* You might need to round the mole ratios to the nearest whole number, but if you're close to a whole number, don't be afraid to round.
* This method only works for compounds made up of two or more elements.
