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Calculate the molecular weight (molar mass) of any chemical compound by entering its elements and atom counts. Essential for stoichiometry, solution preparation, and quantitative chemistry.
Molar mass is the mass of one mole (6.022×10²³ particles) in grams. It equals the sum of atomic masses of all atoms in the formula. This bridges atomic-scale chemistry to lab-scale gram measurements.
Use IUPAC atomic masses to calculate molar mass of any compound. Sum each element atomic mass multiplied by its count in the formula.
| Element | Atomic Mass (g/mol) | Use |
|---|---|---|
| H | 1.008 | Lightest element |
| C | 12.011 | Organic backbone |
| N | 14.007 | Proteins, fertilizers |
| O | 15.999 | Most abundant in crust |
| Na | 22.990 | Table salt |
| S | 32.060 | Amino acids |
| Cl | 35.450 | Salt, bleach |
| Ca | 40.078 | Bones, teeth |
| Fe | 55.845 | Hemoglobin |
| Cu | 63.546 | Electrical wiring |
Water H₂O: 2×1.008 + 15.999 = 18.015 g/mol. NaCl: 22.990 + 35.450 = 58.44 g/mol. Glucose C₆H₁₂O₆: 6×12.011 + 12×1.008 + 6×15.999 = 180.156 g/mol.
Moles = Mass / Molar Mass. 50g NaCl: 50/58.44 = 0.856 mol. Reverse: Mass = Moles × MM. 2 mol NaCl: 2 × 58.44 = 116.88g.
6.022×10²³ particles per mole. One mole of any substance has mass equal to its molar mass in grams. This constant bridges atomic and lab scales.
Molar mass is the most fundamental conversion factor in quantitative chemistry. Every time you prepare a solution, calculate a reaction yield, or interpret a mass spectrum, you are using molar mass. Without it, the bridge between counting atoms and weighing grams would not exist.
The atomic masses on the periodic table are weighted averages of all naturally occurring isotopes. Carbon has two stable isotopes — ¹²C (98.89%) and ¹³C (1.11%) — giving an average atomic mass of 12.011 g/mol rather than exactly 12. This is why molar masses are rarely whole numbers. Chlorine has two abundant isotopes (³⁵Cl and ³⁷Cl), giving it an atomic mass of 35.45 g/mol.
Drug dosing is based entirely on molar mass relationships. Aspirin (acetylsalicylic acid, C₉H₈O₄) has a molar mass of 180.16 g/mol. A 325 mg aspirin tablet contains 325/180.16 = 1.804 mmol of aspirin. Active pharmaceutical ingredients are quantified in moles to ensure consistent receptor occupancy regardless of the molecular size of the drug.
Mass spectrometry directly measures molecular mass by ionizing molecules and separating them by mass-to-charge ratio. The molecular ion peak (M⁺) gives the exact molecular mass. This allows identification of unknown compounds and verification of synthetic products. High-resolution mass spectrometry can determine molecular formulas to four decimal places, distinguishing compounds with the same nominal mass but different elemental compositions.
Molar mass (also called molecular weight) is the mass of exactly one mole (6.022 × 10²³ particles) of a substance, expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of every atom in the chemical formula, weighted by the number of times each element appears. This fundamental quantity connects the microscopic world of atoms to the macroscopic world of grams and liters that chemists measure in the laboratory.
Every quantitative chemistry calculation — preparing solutions, calculating reaction yields, interpreting mass spectra, dosing pharmaceuticals — requires molar mass as the conversion factor between grams and moles. Without it, the bridge between counting atoms and weighing substances would not exist.
Atomic masses on the periodic table are weighted averages across all naturally occurring isotopes of each element. Carbon has two stable isotopes: ¹²C (98.89% abundance, mass 12.000 u) and ¹³C (1.11%, mass 13.003 u). Weighted average: 12.000 × 0.9889 + 13.003 × 0.0111 = 12.011 g/mol. Chlorine's two abundant isotopes (³⁵Cl and ³⁷Cl) give it an atomic mass of 35.45 — explaining why chlorine is not a whole number on the periodic table.
Water (H₂O): 2 × H(1.008) + 1 × O(15.999) = 2.016 + 15.999 = 18.015 g/mol. Glucose (C₆H₁₂O₆): 6×12.011 + 12×1.008 + 6×15.999 = 72.066 + 12.096 + 95.994 = 180.156 g/mol. Sodium chloride (NaCl): 22.990 + 35.450 = 58.440 g/mol. Sulfuric acid (H₂SO₄): 2×1.008 + 32.060 + 4×15.999 = 2.016 + 32.060 + 63.996 = 98.072 g/mol. Caffeine (C₈H₁₀N₄O₂): 8×12.011 + 10×1.008 + 4×14.007 + 2×15.999 = 194.19 g/mol.
Moles = Mass (g) ÷ Molar Mass (g/mol). To prepare 2 moles of NaCl: 2 mol × 58.44 g/mol = 116.88 g. To find moles in 50g of glucose: 50 ÷ 180.16 = 0.278 mol. This conversion is used constantly — from preparing standard solutions in a teaching lab to calculating drug doses in pharmaceutical manufacturing. One mole of any substance contains exactly 6.022 × 10²³ particles (Avogadro number) and has a mass equal to the molar mass in grams.
Drug dosing is calculated using molar mass. Aspirin (acetylsalicylic acid, C₉H₈O₄, MM = 180.16 g/mol): a 325 mg tablet contains 325 ÷ 180.16 = 1.804 mmol of active ingredient. Metformin (C₄H₁₁N₅, MM = 129.16 g/mol): standard 500 mg dose = 3.87 mmol. Pharmaceutical chemists use molar mass to ensure consistent pharmacological activity — the number of molecules delivered to receptors determines drug effect, not just the mass.
Mass spectrometry (MS) measures molecular weight directly by ionizing molecules and separating them by mass-to-charge ratio. The molecular ion peak (M⁺) gives the exact molecular mass. High-resolution MS can determine molecular formulas to 4-5 decimal places — distinguishing compounds with the same nominal mass but different elemental compositions. CO₂ (exact mass 43.9898) and C₂H₄O (exact mass 44.0262) are easily separated at high resolution despite both having nominal mass 44.
H₂O = 18.015, NaCl = 58.440, CO₂ = 44.009, H₂SO₄ = 98.072, HCl = 36.458, NaOH = 39.997, CaCO₃ = 100.086, NH₃ = 17.031, CH₄ = 16.043, C₂H₅OH (ethanol) = 46.068, Glucose C₆H₁₂O₆ = 180.156, Sucrose C₁₂H₂₂O₁₁ = 342.297, Aspirin C₉H₈O₄ = 180.159, Caffeine C₈H₁₀N₄O₂ = 194.194 g/mol. Memorizing the most common ones speeds up chemistry calculations dramatically.