Molecular Weight Calculator
Calculate the molecular weight (molar mass) of any chemical compound. Enter element symbols and atom counts to get g/mol — used for molarity calculations and solution preparation.
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Enter your values above to see the results.
Tips & Notes
- ✓Use the periodic table for atomic weights: H=1.008, C=12.011, N=14.007, O=15.999, Na=22.990, Cl=35.45, S=32.06, P=30.974, Ca=40.078, Fe=55.845. Sum (atomic weight × count) for each element.
- ✓For hydrated salts like CuSO₄·5H₂O, include the water molecules: MW = MW(CuSO₄) + 5 × MW(H₂O) = 159.61 + 5 × 18.015 = 249.69 g/mol. Anhydrous MW gives wrong mass when preparing solutions from hydrated salts.
- ✓IUPAC recommends molar mass (g/mol) rather than molecular weight (dimensionless ratio), but both terms are used interchangeably in laboratory practice. The numerical values are essentially identical for practical purposes.
- ✓For polymers and proteins, average molecular weight varies by sample. Report number-average Mn, weight-average Mw, or viscosity-average Mv depending on the analytical method used — these differ significantly for polydisperse samples.
- ✓Verify your formula by checking element counts match the compound name: glucose C₆H₁₂O₆ has 6+12+6=24 atoms total; aspirin C₉H₈O₄ has 9+8+4=21 atoms. Counting errors are the most common source of wrong molar mass values.
Common Mistakes
- ✗Using integer atomic masses instead of IUPAC standard atomic weights — using 12 for carbon instead of 12.011 introduces 0.09% error per carbon. For a 12-carbon compound like glucose, this is 1.08 g/mol error in 180.16 g/mol total.
- ✗Forgetting water of crystallization in hydrated salts — CuSO₄·5H₂O has MW 249.69 g/mol, not 159.61 g/mol (anhydrous CuSO₄). Using the anhydrous MW to weigh hydrated salt gives 56% more material than intended.
- ✗Confusing molecular formula with empirical formula — empirical formula (CH₂O for glucose) gives the smallest integer ratio, not the actual molecule count. Always use the molecular formula (C₆H₁₂O₆) for molecular weight calculations.
- ✗Misreading subscript numbers in complex formulas — Ca₃(PO₄)₂ has 3 Ca, 2 P, and 8 O (2 × 4). The parentheses multiply all atoms inside: MW = 3(40.078) + 2(30.974) + 8(15.999) = 120.234 + 61.948 + 127.992 = 310.17 g/mol.
- ✗Using molecular weight of the free acid when the salt form is specified — sodium acetate (CH₃COONa) has MW 82.03 g/mol, while acetic acid (CH₃COOH) has MW 60.05 g/mol. Preparing 1 M sodium acetate from acetic acid requires a different calculation entirely.
Molecular Weight Calculator Overview
Molecular weight (molar mass) is the bridge between the microscopic world of atoms and molecules and the macroscopic world of grams and liters in the laboratory. Every quantitative chemical operation — weighing a solid to make a solution, calculating reaction yields, preparing dilutions, running gel electrophoresis — requires knowing the molar mass of the substances involved.
Molecular weight calculation:
MW (g/mol) = Σ (Atomic Weight × Atom Count) for all elements
EX: Calcium phosphate Ca₃(PO₄)₂ → 3×Ca + 2×P + 8×O = 3(40.078) + 2(30.974) + 8(15.999) = 120.234 + 61.948 + 127.992 = 310.17 g/mol. Hydrated: CuSO₄·5H₂O = 159.60 + 5(18.015) = 249.69 g/molConverting mass to moles and back:
Moles = Mass (g) / MW (g/mol) | Mass = Moles × MW (g/mol)
EX: How many moles in 25.0 g of NaOH (MW = 40.00 g/mol)? Moles = 25.0/40.00 = 0.625 mol. To make 0.5 M solution: V = 0.625/0.5 = 1.25 L. Mass of 2 mol glucose = 2 × 180.16 = 360.32 g.IUPAC standard atomic weights — most common elements:
| Element | Symbol | Atomic Weight (g/mol) | Common In |
|---|---|---|---|
| Hydrogen | H | 1.008 | Water, acids, organics |
| Carbon | C | 12.011 | All organic compounds |
| Nitrogen | N | 14.007 | Proteins, nucleic acids |
| Oxygen | O | 15.999 | Water, oxides, organics |
| Sodium | Na | 22.990 | NaCl, NaOH, buffers |
| Phosphorus | P | 30.974 | ATP, DNA, phosphates |
| Sulfur | S | 32.06 | Proteins, H₂SO₄ |
| Chlorine | Cl | 35.45 | HCl, NaCl, organochlorides |
| Potassium | K | 39.098 | KCl, KOH, buffers |
| Calcium | Ca | 40.078 | CaCl₂, Ca(OH)₂, bone |
| Iron | Fe | 55.845 | Hemoglobin, FeCl₃ |
| Copper | Cu | 63.546 | CuSO₄, biochemistry |
| Compound | Formula | MW (g/mol) | Mass for 1 M in 1 L |
|---|---|---|---|
| Water | H₂O | 18.015 | 18.015 g |
| Sodium chloride | NaCl | 58.44 | 58.44 g |
| Sodium hydroxide | NaOH | 40.00 | 40.00 g |
| Hydrochloric acid | HCl | 36.46 | 36.46 g |
| Sulfuric acid | H₂SO₄ | 98.08 | 98.08 g |
| Glucose | C₆H₁₂O₆ | 180.16 | 180.16 g |
| Sucrose | C₁₂H₂₂O₁₁ | 342.30 | 342.30 g |
| EDTA (free acid) | C₁₀H₁₆N₂O₈ | 292.24 | 292.24 g |
| Tris base | C₄H₁₁NO₃ | 121.14 | 121.14 g |
| Copper sulfate pentahydrate | CuSO₄·5H₂O | 249.69 | 249.69 g |
Frequently Asked Questions
Identify all elements and their counts from the chemical formula. Look up the standard atomic weight of each element (from the periodic table or IUPAC values). Multiply each element's atomic weight by its count and sum all values. Example: water H₂O → H: 2 × 1.008 = 2.016; O: 1 × 15.999 = 15.999; Total MW = 18.015 g/mol. For NaCl → Na: 22.990; Cl: 35.45; Total = 58.44 g/mol. For sulfuric acid H₂SO₄ → 2(1.008) + 32.06 + 4(15.999) = 2.016 + 32.06 + 63.996 = 98.07 g/mol.
Molar mass (g/mol) is the mass of one mole of a substance — this is what you calculate from the periodic table and use in laboratory calculations. Molecular mass (Da or u) is the mass of a single molecule in daltons (1 Da = 1 atomic mass unit = 1.66054 × 10⁻²⁴ g). The numerical values are identical: glucose has molar mass 180.16 g/mol and molecular mass 180.16 Da. Molecular weight (technically dimensionless, the ratio of molecular mass to 1 Da) is used interchangeably with molar mass in most laboratory contexts, despite being technically distinct by IUPAC definition.
Add the molar mass of water (18.015 g/mol) for each water molecule in the hydration number. CuSO₄·5H₂O: MW(CuSO₄) = 63.546 + 32.06 + 4(15.999) = 159.60 g/mol. Add 5 × 18.015 = 90.075. Total MW = 159.60 + 90.075 = 249.68 g/mol. This distinction is critical in solution preparation — if you weigh CuSO₄·5H₂O to make 1 M CuSO₄, you need 249.68 g/L, not 159.60 g/L. Using the anhydrous MW would give a solution 57% more concentrated than intended.
Most commonly used in chemistry and biochemistry: H=1.008, C=12.011, N=14.007, O=15.999, S=32.06, P=30.974, Na=22.990, K=39.098, Cl=35.45, Ca=40.078, Mg=24.305, Fe=55.845, Cu=63.546, Zn=65.38, Br=79.904, I=126.90. For calculation shortcuts: carbohydrates (CₙH₂ₙOₙ) have MW = 30n g/mol for the empirical unit; proteins average ~110 Da per amino acid residue; DNA averages ~308 Da per nucleotide. IUPAC updates standard atomic weights periodically — use current values for high-precision work.
Molecular weight converts between mass (grams) and moles: moles = mass / MW; mass = moles × MW. In stoichiometry, this conversion is applied at each step: grams → moles (divide by MW) → use molar ratio from balanced equation → moles product → grams (multiply by MW product). Example: 10 g NaOH reacts with HCl. Moles NaOH = 10/40.00 = 0.250 mol. From NaOH + HCl → NaCl + H₂O (1:1 ratio), 0.250 mol HCl is consumed. Volume of 0.5 M HCl = 0.250/0.5 = 0.500 L = 500 mL. Accurate MW is the foundation of all quantitative chemical calculations.
Approximate molecular weights of common biomolecules: amino acids 75-205 Da (average ~110 Da in proteins); average protein per kDa has ~9 amino acids; ATP = 507 Da; glucose = 180.16 Da; sucrose = 342.30 Da; cholesterol = 386.65 Da; phospholipid (DPPC) = 733.6 Da; DNA nucleotide ~308 Da (average); RNA nucleotide ~330 Da (average). For proteins, estimate MW from sequence: sum all amino acid residues MW and subtract 18 Da per peptide bond (water lost during condensation). SDS-PAGE separates proteins by MW; western blotting identifies proteins by MW using molecular weight markers.