Molar Mass Calculator
Molar mass is the mass of one mole of a substance, expressed in grams per mole, and it is calculated by summing the atomic masses of all atoms in the molecular formula. This calculator handles the four most common elements in organic and biochemical molecules — carbon, hydrogen, oxygen, and nitrogen — making it ideal for quick formula-weight lookups.
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Formula
M = Σ(nᵢ × Aᵢ)
M is the molar mass in g/mol. nᵢ is the number of atoms of element i in the formula. Aᵢ is the standard atomic weight of element i: C = 12.011, H = 1.008, O = 15.999, N = 14.007 g/mol. The molar mass is the sum of all these products across all elements. It connects the microscopic (atomic mass units) scale to the laboratory (grams) scale through Avogadro's number.
How to use the Molar Mass Calculator
- 1
Enter your carbon atoms (c)
Value should be in atoms.
- 2
Enter your hydrogen atoms (h)
Value should be in atoms.
- 3
Enter your oxygen atoms (o)
Value should be in atoms.
- 4
Enter your nitrogen atoms (n)
Value should be in atoms.
- 5
Read your results instantly
Results update in real time as you type.
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Why molar mass is the bridge between grams and moles
Chemists work with both microscopic entities (atoms and molecules) and macroscopic quantities (grams on a balance). Molar mass is the conversion factor between these two scales. One mole of any substance contains 6.022 × 10²³ entities (Avogadro's number). The molar mass tells you the mass of that enormous collection in grams. For water (H₂O), molar mass = 2(1.008) + 15.999 = 18.015 g/mol, so 18.015 grams of water contains exactly one mole of water molecules. This relationship makes molar mass indispensable for stoichiometry, solution preparation, and spectroscopic analysis where you need to know moles from a measured mass.
Standard atomic weights and their precision
Atomic weights are not exact integers because most elements are mixtures of isotopes, each with a slightly different mass. The standard atomic weight of carbon is 12.011 g/mol — not exactly 12 — because natural carbon is about 98.9% carbon-12 and 1.1% carbon-13. IUPAC periodically updates standard atomic weights as measurement precision improves. For most laboratory calculations, using C = 12.011, H = 1.008, O = 15.999, and N = 14.007 provides more than sufficient accuracy. High-resolution mass spectrometry uses monoisotopic masses (the most abundant isotope only) instead, which is why a protein's formula weight from the sequence database may differ slightly from its measured mass spectrum peak.
Tips & Insights
Use subscripts as atom counts
In a formula like C₆H₁₂O₆ (glucose), the subscripts directly give you the atom counts: 6 carbons, 12 hydrogens, 6 oxygens. Enter those numbers directly.
Add other elements manually
This calculator covers C, H, O, and N. For molecules containing phosphorus (P = 30.974), sulfur (S = 32.06), or halogens, add those contributions manually to the result.
Molar mass equals molecular weight in g/mol
The terms 'molar mass,' 'molecular weight,' and 'formula weight' are often used interchangeably in lab contexts, though they have subtle technical differences. For calculation purposes, they give the same number.
Worked Examples
Glucose (C₆H₁₂O₆)
Molar mass = 180.156 g/mol — the standard formula weight of glucose used in biochemistry.
Urea (CH₄N₂O)
Molar mass = 60.055 g/mol — the widely used fertilizer and biochemical reagent.
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Frequently Asked Questions
What is the difference between molar mass and molecular mass?
Molecular mass refers to the mass of a single molecule in atomic mass units (amu or Da). Molar mass is numerically identical but expressed in grams per mole (g/mol) and applies to a mole of molecules.
How do I find the molar mass of an ionic compound?
Use the same approach — sum the atomic masses of all ions in the formula unit. NaCl = 22.990 (Na) + 35.45 (Cl) = 58.44 g/mol.
Why don't atomic masses match atomic numbers exactly?
Atomic number equals protons only. Atomic mass includes the mass of protons, neutrons, and electrons (minus binding energy). Most elements also exist as mixtures of isotopes, giving a weighted average atomic mass.
Can I use this for polymers or proteins?
For small molecules, yes. For large polymers and proteins, molar mass is usually determined experimentally by gel electrophoresis, mass spectrometry, or dynamic light scattering rather than manual atom counting.
What does g/mol mean physically?
It means that many grams of the substance contain exactly one mole (6.022 × 10²³) of formula units. So 18.015 g of water contains 6.022 × 10²³ water molecules.
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