The empirical formula represents the simplest whole numbered ratio of atoms in a compound, while the molecular formula or chemical formula represents the actual ratio of atoms in a compound.
How do you determine the empirical formula?
Questions usually begin with the percent compositions of elements to help find the empirical formula for ionic compounds or covalent compounds.
"A compound is composed of 63.81% carbon, 6.42% hydrogen, and 29.77% nitrogen by mass. Determine the empirical formula of this compound."
STEP 1: Whenever percentages are given we assume that they are out of 100 grams of compound. This allows us to automatically change percentages into grams.
STEP 2: Convert the grams of each element into moles.
To avoid potential rounding errors, make sure you keep at least 3 decimal places for your answers in moles.
STEP 3: Divide all your mole answers by the smallest mole value calculated. Once you’ve divided all the moles by the smallest mole value you should obtain whole numbers.
STEP 4: From division we have 2.5 carbons. This step only occurs when you don’t obtain whole numbers for each element.
We can only round if your value is X.1 or X.9. For example, if you had 2.1 carbons you could round down to 2 carbons. To obtain whole numbers we must multiply all the previous values by the same integer.
STEP 5: With whole numbers representing subscripts for each element we can now obtain the empirical formula.
At this point we have our empirical formula. The additional step that follows is used when we are asked to determine the molecular formula.
STEP 6: For example:
"If the molar mass of the compound is 188.2 g/mol, what would be the molecular formula of the compound?"
To calculate the molecular formula we use the ratio below:
The molecular mass or molar mass of the compound is traditionally given within the question, while the empirical mass is calculated from the empirical formula.
Calculating Molecular Mass to Empirical Mass Ratio
Plugging in the molecular mass and empirical mass into the ratio will provide us with a whole number integer. To determine the molecular formula you will multiply the empirical formula by that integer.
Beyond calculating the empirical and molecular formulas you can also deal with steps involving combustion analysis.