The molecular formula represents the true ratio of atoms, whose subscripts show the actual number of each type of atom, in a given chemical compound. It is closely linked to the molar mass or molecular mass of a chemical compound.
Molecular Formula vs. Empirical Formula
Determining the molecular formula of a chemical compound usually first involves finding its empirical formula, which represents the simple or condensed formula.
Tetraphosphorus hexoxide (P4O6) represents the true formula for the chemical compound. Now since both 4 and 6 are divisible by 2 we can reduce the molecular formula to diphosphorus trixoxide (P2O3).
The same method of converting the molecular formulas of caffeine and butane can be applied to determine their empirical formulas.
Determining the Molecular Formula
To determine the molecular formula of a chemical compound we must first determine its empirical formula.
For example, if we are asked “A 3.927 sample of a compound was found to contain 1.536 g carbon, 0.345 g hydrogen, and the rest as oxygen. If the molecular mass of the compound is 184.18 g/mol, what is the molecular formula?” These are the steps that would need to be followed:
STEP 1: Find the mass (in grams of oxygen) by subtracting out the masses of carbon and hydrogen from the total mass of the compound.
3.927 g sample – (1.536 g C + 0.345 g H) = 2.046 g O
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 the moles you found in Step 2 by the smallest mole value you found to determine the empirical ratios.
STEP 4: All the empirical ratios obtained from Step 3 must be whole numbers. You can only round an empirical ratio value if it’s X.1 or X.9. For example, if you obtained 2.9 hydrogens you could round up to 3.0 hydrogens.
Multiply all the empirical ratios by the same value in order to obtain whole numbers.
STEP 5: Use your new empirical ratios, which are now all whole numbers, to determine the empirical formula.
STEP 6: We must determine an “n” value by using the following equation:
The molecular mass or molar mass of the compound will be given within the question, but the empirical mass will be calculated from the empirical formula.
Now that you have your “n” value you can multiply the empirical formula by it to isolate your molecular formula.
Empirical and molecular formulas help to determine the structural formulas of compounds. In order to accomplish the calculations needed it will important to understand the nomenclature of these compounds and the mass conversions involved.