What Is Molecular Weight?
Molecular weight, also called molecular mass or molar mass, is the sum of the atomic weights of all atoms in a molecule. It is expressed in grams per mole (g/mol) and is a fundamental property used in chemistry to convert between mass and moles of a substance.
For example, water (H2O) consists of two hydrogen atoms (each weighing 1.008 g/mol) and one oxygen atom (15.999 g/mol), giving a molecular weight of 18.015 g/mol. This means one mole of water weighs 18.015 grams.
What This Calculator Does
This calculator takes a chemical formula and computes the molecular weight by looking up the atomic weight of each element from the periodic table and summing their contributions.
- Input: A chemical formula using standard element symbols (e.g. H2O, C6H12O6, NaCl)
- Output: Total molecular weight in g/mol, with a breakdown of each element's contribution and mass percentage
How the Calculation Works
Molecular Weight = sum of (Atomic Weight × Count) for each element
The formula is parsed element by element. Each element symbol (one or two letters, capitalized) is matched to its atomic weight from the periodic table. The subscript number following the symbol gives the count. If no number follows, the count is 1.
For glucose (C6H12O6): Carbon: 12.011 × 6 = 72.066. Hydrogen: 1.008 × 12 = 12.096. Oxygen: 15.999 × 6 = 95.994. Total: 180.156 g/mol.
How to Use the Calculator
- Type the chemical formula in the input field
- Use uppercase for the first letter of each element symbol and lowercase for the second letter (e.g. Na, Cl, Fe)
- Follow each element with a number if more than one atom is present
- Use the quick example buttons to load common compounds
- The molecular weight and element breakdown appear instantly
Example Calculations
Example 1: Sulfuric acid (H2SO4)
H: 1.008 × 2 = 2.016. S: 32.065 × 1 = 32.065. O: 15.999 × 4 = 63.996. Total: 98.077 g/mol.
Example 2: Aspirin (C9H8O4)
C: 12.011 × 9 = 108.099. H: 1.008 × 8 = 8.064. O: 15.999 × 4 = 63.996. Total: 180.159 g/mol.
Real-World Scenarios
Preparing Chemical Solutions
To prepare a 1 M solution of NaCl, you need to weigh out one mole of NaCl. The molecular weight of NaCl (22.990 + 35.453 = 58.443 g/mol) tells you to weigh exactly 58.443 grams per liter of solution.
Stoichiometry Calculations
Chemical reactions involve mole ratios. To calculate how many grams of a reactant or product are involved, you need the molecular weight to convert between grams and moles. For example, 2H2 + O2 gives 2H2O. Burning 4.032 g of H2 (2 × 2.016) produces 36.03 g of water (2 × 18.015).
Pharmaceutical and Food Chemistry
Nutritional labels express calorie content using the molecular weights of carbohydrates (roughly 4 kcal/g), proteins (4 kcal/g), and fats (9 kcal/g). Drug dosing often depends on the molecular weight to determine how many molecules fit in a therapeutic dose.
Why This Calculation Matters
Molecular weight is essential for any quantitative chemistry work. It is the bridge between the macroscopic world of grams you can weigh and the microscopic world of atoms and molecules that actually react. Without it, you cannot perform stoichiometry, prepare standard solutions, or interpret chemical analyses.
Common Mistakes to Avoid
- Wrong capitalization: Co is cobalt (58.933 g/mol), while CO is carbon monoxide (12.011 + 15.999 = 28.01 g/mol). Always capitalize only the first letter of the element symbol
- Hydrated salts: CuSO4 has a different molecular weight than CuSO4·5H2O (anhydrous vs. hydrated). Make sure you use the correct form of the compound
- Parentheses in complex formulas: This calculator handles simple linear formulas. For compounds with parentheses like Ca(OH)2, expand the formula manually to CaO2H2 before entering it