Ch.2 - Atoms & ElementsSee 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

Molar mass or molecular weight is a physical property that represents the mass of a substance divided by the amount of that substance. The common units are typically in grams per mole (g/mol), while the si units are in kilograms per mole (kg/mol). 

How to Calculate Molar Mass

In order to determine the molar mass of a compound we must determine the number of atoms for the elements within a given compound. 

For example, if we are asked to determine the molar mass of glucose, C6H12O6, we can solve with steps shown below. 

STEP 1: Count the number of each element within the compound to get 6 carbons, 12 hydrogens and 6 oxygens. 

Number-of-AtomsNumber of atoms 

STEP 2: Find the atomic mass for each chemical element from the periodic table. 

Periodic-Table-Atomic -MassesPeriodic Table (Atomic Masses)

STEP 3: Multiply the number of each element by their atomic masses taken from the periodic table.

Addition-Atomic-Masses-GlucoseAddition of Atomic Masses

STEP 4: Add up all the masses you got from multiplication together to give the overall molar mass of the compound. 

Sum-Total-Molecular-Compound-GlucoseMolar Mass of Glucose

From simple compounds such as CO2, H2O, or ammonia to more complex compounds such as proteins, amino acids, acetone or acetic acid we now calculate their molar masses. 

PRACTICE 1: Calculate the formula weight of aspirin, C9H8O4, based on the given molecular formula. 

Weight-Table-AspirinMolar Mass (Aspirin)

Aspirin contains 9 carbon, 8 hydrogen and 4 oxygen atoms with a total mass of 180.154 g/mol. 

PRACTICE 2: Calculate the molar mass of silver nitrate, AgNO3.

Molar Mass (Silver Nitrate)

Silver nitrate contains 1 silver, 1 nitrogen and 3 oxygen atoms with a total mass of 169.88 g/mol. 

PRACTICE 3: Determine the molecular weight of ammonium phosphate, (NH4)3PO4

Molar Mass (Ammonium Phosphate)Molar Mass (Ammonium Phosphate)

Ammonium phosphate contains 3 nitrogens, 12 hydrogens, 1 phophate and 4 oxygen atoms with a total mass of 149.096 g/mol. 

Why is the molar mass so important? 

Molar mass serves as the foundation for many calculations and ideas found in chemistry. It serves as the bridge that connects together concepts in stoichiometry such as mass conversions, moles, avogadro’s number (6.022 x 1023) or in dimensional analysis such as density, amu, and moles. 


In addition we discuss the average molar mass or average molecular mass of compounds when we cover isotopes, the colligative properties such as boiling point elevation or freezing point depression, and the rates of gases through effusion and diffusion. 


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.