How to Calculate Molar Mass — Step-by-Step with Examples

By Aniket Bhardwaj · 27 August 2026 · Calculator/Formula Guide

Molar mass is the first calculation every chemistry student must master. It appears in stoichiometry, solution preparation, gas law problems and almost every numerical you will meet from Class 11 to CSIR-NET. This guide shows the exact method, four fully worked examples, and the mistakes that cost students marks in exams.

What is molar mass?

Molar mass is the mass of one mole (6.022 × 10²³ particles) of a substance, expressed in grams per mole (g/mol). Numerically it equals the sum of the atomic masses of every atom in the formula.

Molar mass M = Σ (number of atoms of each element × atomic mass of that element)

Atomic masses come from the periodic table: H = 1.008, C = 12.011, O = 15.999, K = 39.098, Cr = 51.996, Ca = 40.078 (all in g/mol, rounded to three decimals).

The 3-step method

List every element in the formula and count its atoms — expand brackets carefully: Ca(OH)₂ contains 1 Ca, 2 O and 2 H.
Multiply each element's atom count by its atomic mass.
Add all the products. The total is the molar mass in g/mol.

Worked example 1 — Water, H₂O

H: 2 × 1.008 = 2.016
O: 1 × 15.999 = 15.999
M(H₂O) = 2.016 + 15.999 = 18.015 ≈ 18.02 g/mol

Worked example 2 — Carbon dioxide, CO₂

C: 1 × 12.011 = 12.011
O: 2 × 15.999 = 31.998
M(CO₂) = 12.011 + 31.998 = 44.009 ≈ 44.01 g/mol

Worked example 3 — Potassium dichromate, K₂Cr₂O₇

This is a favourite in board exams and JAM/NET papers because it tests careful counting.

K: 2 × 39.098 = 78.196
Cr: 2 × 51.996 = 103.992
O: 7 × 15.999 = 111.993
M(K₂Cr₂O₇) = 78.196 + 103.992 + 111.993 = 294.18 g/mol

Worked example 4 — Calcium hydroxide, Ca(OH)₂ (brackets!)

The subscript 2 outside the bracket multiplies everything inside:
Ca: 1 × 40.078 = 40.078
O: 2 × 15.999 = 31.998
H: 2 × 1.008 = 2.016
M(Ca(OH)₂) = 40.078 + 31.998 + 2.016 = 74.09 g/mol

Percent composition — the natural next step

Once you have the molar mass, the mass percentage of each element is:

% element = (mass contributed by that element ÷ molar mass) × 100

For water: %H = (2.016 ÷ 18.015) × 100 = 11.19% and %O = (15.999 ÷ 18.015) × 100 = 88.81%. The two must add to 100 — a built-in check for your working.

Common mistakes that cost marks

Forgetting bracket multiplication: writing Ca(OH)₂ as Ca + O + H₂ gives 57.09 instead of 74.09.
Using mass number instead of atomic mass: Cl is 35.45 g/mol (weighted average of isotopes), not 35.
Confusing molar mass with molecular mass: molecular mass is in atomic mass units (u) for one molecule; molar mass is in g/mol for one mole. Same number, different meaning and unit.
Rounding too early: keep 3 decimals during the working; round only the final answer to 2 decimals.

Where molar mass appears in exams

Exam	Typical use
CBSE/ICSE Class 11–12	Mole concept, concentration of solutions, stoichiometry
JEE/NEET	Limiting reagent, empirical formula problems
IIT-JAM / CUET-PG	Solution preparation, colligative properties
GATE / CSIR-NET	Equivalent weight, titration and gravimetric calculations

Check your answer instantly. The free Molar Mass & Composition calculator accepts any formula — K2Cr2O7, CuSO4.5H2O, even organic formulas — and shows the element-wise breakdown and percent composition.

Open the Molar Mass Calculator →

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