Ch.3 - Chemical ReactionsSee all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds
Jules Bruno

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. 

Molecular-Formula-Empirical-Formula-Structural-FormulaMolecular Formula vs. Empirical 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). 

Caffeine-butane-water-worksheet-empirical-formula-molecular-formula-definitionCaffeine & Butane (Molecular Formulas & Empirical Formulas)

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. 

Mass-conversions-C-H-O-molesMass Conversions (Grams C, H, O to 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. 

Smallest-moles-subscripts-actual-molecule-representDividing by the smallest moles

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. 

Empirical-Ratios-RoundingEmpirical Ratios & Rounding

Multiply all the empirical ratios by the same value in order to obtain whole numbers. 

Empirical-ratios-whole-numbers-multiplicationEmpirical Ratios, Multiplication & Whole Numbers

STEP 5: Use your new empirical ratios, which are now all whole numbers, to determine the empirical formula. 

Empirical-Formula-C3H8O3Empirical Ratios & the Empirical Formula (C3H8O3)

STEP 6: We must determine an “n” value by using the following equation: 

Molecular-mass-divided-division-empirical-formulaMolecular Mass divided by Empirical Mass

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. 

Calculating-molecular-massCalculating molecular mass (C3H8O3)

Now that you have your “n” value you can multiply the empirical formula by it to isolate your molecular formula. 

Determining-molecular-formula-C3H8O3Determining Molecular Formula (C3H8O3)

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. 

Jules Bruno

Jules felt a void in his life after his English degree from Duke, so he started tutoring in 2007 and got a B.S. in Chemistry from FIU. He’s exceptionally skilled at making concepts dead simple and helping students in covalent bonds of knowledge.