Molar mass links grams and moles in every chemistry calculation. Type a formula such as H2SO4, Ca(OH)2 or CuSO4·5H2O to get its molar mass and the percentage of each element by mass.
Type the chemical formula with correct capital letters.
Use · or . for hydrates.
Read the molar mass and composition.
Molar mass: M = Σ (atoms of element × atomic mass)
Molar mass is the mass of one mole of a substance, in grams per mole. This calculator reads a chemical formula you type, counts every atom including those inside brackets and water of crystallisation, multiplies each by its standard atomic mass and adds them up. It also shows the percentage by mass of each element in the compound.
Nearly every numerical in Class 11 and 12 chemistry passes through molar mass, because it is the bridge between grams, which you weigh on a balance, and moles, which chemical equations count. It is used when preparing solutions, balancing stoichiometry, finding empirical formulas and checking fertiliser labels. Getting it right first saves many downstream errors in JEE and NEET problems.
1. Write the formula with correct capitals, for example NaCl, H2SO4, Ca(OH)2.
2. Count atoms of each element. A subscript after a bracket multiplies everything inside it; in Ca(OH)2 there are 2 O and 2 H.
3. For a hydrate such as CuSO4·5H2O, add the atoms from the water part, multiplied by its leading number.
4. Multiply each count by the element's atomic mass from the periodic table.
5. Add the products to get the molar mass: M = Σ (number of atoms × atomic mass), in g/mol.
6. For percentage composition, divide each element's contribution by M and multiply by 100.
Atomic masses are measured in unified atomic mass units, u, defined so that one carbon-12 atom is exactly 12 u. The mole is chosen so that one mole of particles in grams equals the particle's mass in u. So a water molecule of about 18.02 u means one mole of water has a mass of about 18.02 g. That is why molar mass in g/mol and molecular mass in u have the same number. The link is Avogadro's number, about 6.022 × 10²³.
Most elements occur as mixtures of isotopes. Chlorine is roughly three-quarters chlorine-35 and one-quarter chlorine-37, so its average atomic mass is about 35.45, not a whole number. The table values used here are these natural averages, rounded to around four or five significant figures. Very precise work, such as mass spectrometry of a single molecule, uses exact isotopic masses instead. For school and lab calculations, standard atomic weights are the right choice.
Brackets group atoms that behave as a unit, like the hydroxide ion in Ca(OH)2 or the sulphate ion in Al2(SO4)3. Hydrates carry water molecules in their crystal, shown with a dot. Blue vitriol, CuSO4·5H2O, has a molar mass of about 249.7 g/mol, of which about 36% is water. The mass percentages the calculator shows are the same numbers used in reverse to find an empirical formula, and to check whether a product label matches its claimed composition.
Shreya needs the molar mass of blue copper sulphate crystals, CuSO4·5H2O, before weighing them for a Class 11 practical.
Atoms × atomic mass: Cu: 1 × 63.546 = 63.546 S: 1 × 32.06 = 32.06 O: 9 × 15.999 = 143.991 H: 10 × 1.008 = 10.08
Molar mass: 63.546 + 32.06 + 143.991 + 10.08 = 249.677 g/mol
Mass percentage: Cu 25.45%, S 12.84%, O 57.67%, H 4.04%
Answer: Molar mass 249.677 g/mol; Composition by mass Cu 25.5%, S 12.8%, O 57.7%, H 4%
Typing CO for cobalt or NACL for sodium chloride; capital letters change the element.
Forgetting to multiply everything inside a bracket by the subscript outside it.
Using the anhydrous molar mass for a hydrated salt.
Applying a leading coefficient from a balanced equation, like the 2 in 2H2O, as part of the molar mass.
Rounding atomic masses to whole numbers too early in precise work.
Converting grams to moles in Class 11 and 12 chemistry problems.
Preparing solutions of a required molarity in the lab.
Working out percentage composition for empirical formula questions.
Checking nutrient percentages in fertilisers such as urea and DAP.
Pharmacy and industrial calculations that convert between mass and amount of substance.
Why does capitalisation matter?
Co is cobalt but CO is carbon monoxide.
Which atomic masses are used?
Standard IUPAC values rounded to about four significant figures.