⚗️ Molar Mass Calculators

Calculate molar mass, molecular weight, empirical formulas, and percent composition.

All Molar Mass Tools

Molar Mass Calculator Calculate the molar mass of any chemical compound from its molecular formula. Molecular Weight Calculator Find the molecular weight of organic and inorganic compounds by formula. Empirical Formula Calculator Find the simplest whole-number ratio of elements from percent composition. Percent Composition Calculator Calculate mass percent of each element in a chemical compound. Moles to Grams Calculator Convert between moles and grams using the molar mass of any compound.

What Is Molar Mass?

Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). One mole contains 6.022 × 10²³ particles (Avogadro's number). The molar mass of an element equals its atomic mass in grams per mole — carbon has a molar mass of 12.011 g/mol, oxygen 15.999 g/mol, hydrogen 1.008 g/mol. For a compound, the molar mass is the sum of the atomic masses of all atoms in one formula unit. Water (H₂O): 2(1.008) + 15.999 = 18.015 g/mol. Glucose (C₆H₁₂O₆): 6(12.011) + 12(1.008) + 6(15.999) = 180.156 g/mol.

Moles, Grams, and Molecular Weight

The mole is the bridge between atomic-scale chemistry and laboratory-scale measurements. The conversion formula is: moles = mass (g) ÷ molar mass (g/mol), or equivalently, mass (g) = moles × molar mass. To convert 25 g of water to moles: 25 ÷ 18.015 = 1.388 mol. To find the mass of 3.5 mol of NaCl (molar mass 58.44 g/mol): 3.5 × 58.44 = 204.5 g. Molecular weight and molar mass are numerically equal but differ in units: molecular weight is dimensionless (expressed in unified atomic mass units, u), while molar mass is expressed in g/mol.

Empirical vs. Molecular Formula

The empirical formula gives the simplest whole-number ratio of atoms in a compound. The molecular formula gives the actual number of atoms per molecule. Glucose has the molecular formula C₆H₁₂O₆ and the empirical formula CH₂O. To determine an empirical formula from percent composition, convert each percentage to grams (assume 100 g sample), divide by atomic mass to get moles, then divide all mole values by the smallest to find the ratio. If the ratios are not whole numbers, multiply by the smallest integer that makes them whole.

Percent Composition by Mass

Percent composition expresses the mass fraction of each element in a compound as a percentage. For element X in compound Y: % X = (mass of X in one mole of Y ÷ molar mass of Y) × 100. For water: % H = (2 × 1.008 ÷ 18.015) × 100 = 11.19%; % O = (15.999 ÷ 18.015) × 100 = 88.81%. Percent composition is used in analytical chemistry to verify compound identity, calculate elemental analysis results, and determine empirical formulas from combustion analysis data.