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Molar Mass from Formula

Calculate the molar mass of any chemical compound from elemental atomic masses and atom counts.

Total Molar Mass

18.015 g/mol

Total Atom Count

3 atoms

Step-by-step working

Calculate elemental contributions: Element 1 = 2 × 1.008 = 2.016 g/mol; Element 2 = 1 × 15.999 = 15.999 g/mol.
Sum all element mass contributions: M = 2.016 + 15.999 + 0 + 0 = 18.015 g/mol.

Elemental Mass Breakdown
Component	Atomic Mass	Count	Subtotal Mass
Element 1	1.008 g/mol	2	2.016 g/mol
Element 2	15.999 g/mol	1	15.999 g/mol

Mathematical Formula

M = Σ (n_i × M_i)

Overview & Explanation

The molar mass of a chemical compound represents the mass in grams of one mole of that substance (g/mol). It serves as a fundamental physical property connecting microscopic molecular quantities with macroscopic laboratory measurements. By summing the standard atomic weights of every constituent atom in a chemical formula, chemists can determine the exact molar mass needed for solution preparation, chemical reactions, and gas stoichiometry.

How It Works

Identify each distinct element present in the chemical formula along with its standard atomic mass (M_i) in grams per mole (g/mol) from the periodic table.
Multiply the atomic mass of each element by its corresponding subscript or stoichiometric count (n_i) in the formula.
Sum the individual mass contributions of all elements together to obtain the total compound molar mass (M).

Practical Applications

Stoichiometric Calculations: Converting measured mass in grams to moles to determine limiting reagents and theoretical yields.
Solution Preparation: Calculating the mass of solid solute required to make a molar solution of specific concentration and volume.
Gas Law Applications: Deriving gas density or applying the ideal gas equation (PV = nRT) in gas stoichiometry calculations.

Water (H2O)

Calculate the molar mass of pure water containing 2 hydrogen atoms and 1 oxygen atom.

Inputs

Element 1 Atomic Mass (g/mol): 1.008 · Element 1 Atom Count: 2 · Element 2 Atomic Mass (g/mol): 15.999 · Element 2 Atom Count: 1 · Element 3 Atomic Mass (g/mol): 0 · Element 3 Atom Count: 0 · Element 4 Atomic Mass (g/mol): 0 · Element 4 Atom Count: 0

= entered

Step 1

Calculate elemental contributions: Element 1 = 2 × 1.008 = 2.016 g/mol; Element 2 = 1 × 15.999 = 15.999 g/mol.

Step 2

Sum all element mass contributions: M = 2.016 + 15.999 + 0 + 0 = 18.015 g/mol.

Total Molar Mass

Result

= 18.015 g/mol

Sulfuric Acid (H2SO4)

Calculate the molar mass of sulfuric acid containing 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms.

Inputs

Element 1 Atomic Mass (g/mol): 1.008 · Element 1 Atom Count: 2 · Element 2 Atomic Mass (g/mol): 32.06 · Element 2 Atom Count: 1 · Element 3 Atomic Mass (g/mol): 15.999 · Element 3 Atom Count: 4 · Element 4 Atomic Mass (g/mol): 0 · Element 4 Atom Count: 0

= entered

Step 1

Calculate elemental contributions: Element 1 = 2 × 1.008 = 2.016 g/mol; Element 2 = 1 × 32.06 = 32.06 g/mol.

Step 2

Sum all element mass contributions: M = 2.016 + 32.06 + 63.996 + 0 = 98.072 g/mol.

Total Molar Mass

Result

= 98.072 g/mol

How it's calculated & sources

M = Σ (n_i × M_i)

NIST Periodic Table of Elements

Frequently Asked Questions

What is the difference between molar mass and molecular weight?

Molar mass is the mass of one mole of a substance expressed in grams per mole (g/mol). Molecular weight (or relative molecular mass) is a dimensionless ratio comparing the mass of a molecule relative to 1/12th the mass of a carbon-12 atom. Numerically, both values are identical in standard laboratory chemistry.

Why do atomic masses on the periodic table have decimal places?

Atomic masses are weighted averages of all naturally occurring isotopes of an element based on their natural abundances. For instance, naturally occurring chlorine consists of ~75.8% Cl-35 and ~24.2% Cl-37, resulting in an average atomic mass of approximately 35.45 g/mol.

How do hydrates affect molar mass calculations?

Hydrates contain water molecules bound within their crystal structure (e.g., CuSO4 · 5H2O). When calculating the molar mass of a hydrate, you must include the full atomic weight contribution of all hydrogen and oxygen atoms present in the hydration water molecules.

What standard unit is used for molar mass?

The standard SI derived unit for molar mass is kilograms per mole (kg/mol), but in practical chemistry and laboratory settings, grams per mole (g/mol) is universally preferred.