Molar Mass Formula
Reference for molar mass in g/mol by summing atomic masses.
Includes examples for H2O (18.02 g/mol), glucose (180.16), NaCl (58.44), and mole conversions.
The Formula
Molar mass is the mass of one mole (6.022 × 10²³ particles) of a substance. It is expressed in grams per mole (g/mol).
To find it, add up the atomic masses of every atom in the chemical formula. Atomic masses are found on the periodic table.
Variables
| Symbol | Meaning |
|---|---|
| M | Molar mass (g/mol) |
| n | Number of atoms of each element in the formula |
| A | Atomic mass of each element (from periodic table, in amu) |
Common Atomic Masses
| Element | Symbol | Atomic Mass (amu) |
|---|---|---|
| Hydrogen | H | 1.008 |
| Carbon | C | 12.011 |
| Nitrogen | N | 14.007 |
| Oxygen | O | 15.999 |
| Sodium | Na | 22.990 |
| Chlorine | Cl | 35.453 |
| Calcium | Ca | 40.078 |
| Iron | Fe | 55.845 |
Example 1 — Water (H₂O)
Find the molar mass of water (H₂O)
Hydrogen: 2 × 1.008 = 2.016
Oxygen: 1 × 15.999 = 15.999
M = 2.016 + 15.999 = 18.015 g/mol
Example 2 — Table Salt (NaCl)
Find the molar mass of sodium chloride (NaCl)
Sodium: 1 × 22.990 = 22.990
Chlorine: 1 × 35.453 = 35.453
M = 22.990 + 35.453 = 58.443 g/mol
Example 3 — Glucose (C₆H₁₂O₆)
Find the molar mass of glucose (C₆H₁₂O₆)
Carbon: 6 × 12.011 = 72.066
Hydrogen: 12 × 1.008 = 12.096
Oxygen: 6 × 15.999 = 95.994
M = 72.066 + 12.096 + 95.994 = 180.156 g/mol
When to Use It
- Converting between grams and moles in chemistry problems
- Calculating the amount of reactants needed for a chemical reaction
- Determining concentration of solutions (molarity = moles / liters)
- Stoichiometry — balancing chemical equations in terms of mass
- Pharmaceutical dosing calculations
Converting Between Grams and Moles
For example, 36.03 grams of water = 36.03 / 18.015 = 2.0 moles of water.
Key Notes
- Definition: molar mass (g/mol) = sum of all atomic masses in the formula: From the periodic table, each element's standard atomic weight is the molar mass contribution per atom. H₂O: 2(1.008) + 15.999 = 18.015 g/mol. CO₂: 12.011 + 2(15.999) = 44.009 g/mol.
- Moles conversion: n = mass / M: The molar mass M serves as the conversion factor between grams and moles. 88.0 g of CO₂: n = 88.0 / 44.009 = 2.00 mol. This conversion is the entry point for all stoichiometry calculations.
- Atomic masses are weighted isotopic averages: Chlorine's molar mass ≈ 35.45 g/mol because natural chlorine is ~75.8% ³⁵Cl (34.97 g/mol) and ~24.2% ³⁷Cl (36.97 g/mol). The periodic table value reflects this natural abundance average — not any single isotope.
- Empirical vs molecular formula: The empirical formula gives the simplest whole-number ratio of atoms. The molecular formula is n × empirical formula where n = molecular molar mass / empirical formula molar mass. Glucose: empirical CH₂O (30 g/mol); molecular C₆H₁₂O₆ (180 g/mol); n = 6.
- Applications: Molar mass is used in every stoichiometry calculation, preparing solutions of known molarity, gas density calculation (M/22.4 g/L at STP), determining the formula of an unknown compound from elemental analysis, and pharmaceutical dosing (mg of drug = moles × M × 1000).