Science 2026-05-04 3 min read By Konvertibly Team

Molar Mass Calculator: Molecular Weight and Mole Calculations

Calculate molar mass of chemical compounds from atomic masses, convert between mass and moles, and find the number of molecules using Avogadro's number.

Molar Mass and Mole Calculations

The molar mass (g/mol) converts between the mass of a substance and the number of moles — the chemist's counting unit for atoms and molecules.

Calculating Molar Mass

M = Σ (number of atoms × atomic mass)

Water (H₂O):
2 × H (1.008) + 1 × O (16.00) = 18.016 g/mol

Glucose (C₆H₁₂O₆):
6×12.011 + 12×1.008 + 6×16.00 = 180.16 g/mol

Mole Conversions

n = m / M        (moles from mass)
m = n × M        (mass from moles)
N = n × Nₐ       (number of molecules)
Nₐ = 6.022×10²³ mol⁻¹ (Avogadro's number)

100g of NaCl (M=58.44 g/mol):
n = 100/58.44 = 1.71 mol
N = 1.71 × 6.022×10²³ = 1.03×10²⁴ molecules

Common Molar Masses (g/mol)

H₂O (water): 18.02
CO₂ (carbon dioxide): 44.01
NaCl (salt): 58.44
C₆H₁₂O₆ (glucose): 180.16
CaCO₃ (calcium carbonate): 100.09
H₂SO₄ (sulfuric acid): 98.08

Calculate molar mass: Free Molar Mass Calculator

Common Molar Masses Quick-Reference Table

Compound	Formula	Molar Mass (g/mol)
Water	H₂O	18.015
Carbon dioxide	CO₂	44.010
Glucose	C₆H₁₂O₆	180.16
Sodium chloride	NaCl	58.44
Ethanol	C₂H₅OH	46.07
Caffeine	C₈H₁₀N₄O₂	194.19
Aspirin	C₉H₈O₄	180.16

How Molar Mass Works

Molar mass is the mass of one mole (6.022×10²³ units) of a substance, in grams per mole. It numerically equals the substance's atomic or molecular weight in atomic mass units (u). For an element, molar mass = atomic weight from the periodic table. For a compound, sum the atomic masses of all atoms: M(H₂O) = 2(1.008) + 15.999 = 18.015 g/mol.

Molar mass converts between the macroscopic (grams) and microscopic (molecules, atoms) worlds. n = m/M gives moles from mass; N = n × Nₐ gives number of molecules. In pharmaceutical manufacturing, precise molar mass calculations determine drug dosages. In stoichiometry, it allows calculation of theoretical yields from balanced equations.

Common Mistakes

Using integer atomic masses: H = 1, O = 16 are rough approximations. Use 1.008 and 15.999 for precise results (especially in analytical chemistry).
Miscounting atoms in complex formulas: Ca(OH)₂ has 1 Ca, 2 O, and 2 H — the subscript outside the parenthesis multiplies everything inside.
Confusing molar mass with molarity: Molar mass is g/mol (a property of a substance). Molarity is mol/L (a concentration — depends on how much is dissolved in solution).

Frequently Asked Questions

Q: What is the difference between molar mass and molecular weight?

They are numerically identical but differ in units: molecular weight (or relative molecular mass) is dimensionless (the ratio of molecular mass to 1/12 of C-12 mass). Molar mass has units of g/mol. In practice, chemists use them interchangeably — both equal the sum of atomic masses in the formula.

Q: How do I calculate the number of molecules in a sample?

N = (m/M) × Nₐ, where m is the sample mass in grams, M is molar mass in g/mol, and Nₐ = 6.022×10²³ mol⁻¹ (Avogadro's number). Example: 18 g of water = 1 mol = 6.022×10²³ molecules. 9 g = 0.5 mol = 3.011×10²³ molecules.

Q: Why do isotopes affect molar mass?

Natural elements are mixtures of isotopes with different masses. The standard atomic weight is the weighted average: chlorine is 75.77% ³⁵Cl (34.969 u) and 24.23% ³⁷Cl (36.966 u), giving a weighted average of 35.45 g/mol. Stable isotope ratios can shift slightly by geography, which is why ultra-precise analytical chemistry uses isotope-specific masses.