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Molar Mass Calculator: How to Calculate Molecular Weight for Any Chemical Compound

Blog › Math · 8 min read · Published 2026-05-05

A complete guide to molar mass — the formula, worked examples for water, salt, glucose and Ca(OH)₂, and the role of molar mass in stoichiometry.

What Is Molar Mass?

Molar mass is the mass in grams of one mole of a substance — that is, the mass of 6.022 × 10²³ particles (Avogadro's number). It's measured in grams per mole (g/mol) and is the bridge between the lab bench (where we weigh things in grams) and the equation sheet (where we count things in moles).

Computing it manually is straightforward but tedious; the Molar Mass Calculator parses any formula — including parentheses — and shows the per-element breakdown.

The Periodic Table Connection

Every element's atomic mass on the periodic table is, by convention, its molar mass in g/mol. Hydrogen is 1.008 g/mol, oxygen is 15.999 g/mol, carbon is 12.011 g/mol. To get a compound's molar mass, you sum the atomic masses of every atom in its formula.

Step-by-Step Examples

Water (H₂O)

2 hydrogens × 1.008 + 1 oxygen × 15.999 = 2.016 + 15.999 = 18.015 g/mol. So one mole of water weighs about 18 g — roughly a tablespoon.

Sodium chloride (NaCl)

22.990 + 35.453 = 58.443 g/mol. To make a 1 M NaCl solution in 1 L of water, dissolve 58.443 g of salt.

Glucose (C₆H₁₂O₆)

6(12.011) + 12(1.008) + 6(15.999) = 72.066 + 12.096 + 95.994 = 180.156 g/mol.

Calcium hydroxide (Ca(OH)₂)

The parentheses mean "everything inside, multiplied by 2." So you get: 40.078 + 2(15.999 + 1.008) = 40.078 + 34.014 = 74.092 g/mol.

Handling Parentheses Correctly

Parentheses with a subscript distribute multiplication across every element inside. Common stumbles:

  • Al₂(SO₄)₃ has 2 aluminum, 3 sulfur (3 × 1), and 12 oxygen (3 × 4).
  • Cu(NO₃)₂ has 1 copper, 2 nitrogen, 6 oxygen.
  • Nested groups like K₃[Fe(CN)₆]: 3 K, 1 Fe, 6 C, 6 N.

Why Molar Mass Matters in Stoichiometry

Chemical equations balance in moles, not grams. Molar mass is what lets you convert between the two. To react 5 g of NaOH with HCl, you first convert: 5 / 40.0 = 0.125 mol NaOH. The reaction is 1:1, so you need 0.125 mol HCl, which equals 0.125 × 36.46 = 4.56 g.

Common Mistakes Students Make

  1. Confusing Co (cobalt) with CO (carbon monoxide). Capitalization is meaningful in chemical formulas.
  2. Forgetting subscripts inside parentheses. Ca(OH)₂ is 74, not 57.
  3. Using rounded atomic masses inconsistently. Stick to four significant figures throughout a calculation.
  4. Mixing up molar mass and formula mass for ionic compounds. They're the same number, just different language.
  5. Forgetting water of hydration. CuSO₄·5H₂O includes 5 water molecules in the molar mass (249.69, not 159.61).

Molar Mass vs Molecular Weight

Numerically identical. "Molecular weight" is dimensionless (a relative comparison to ¹²C); "molar mass" carries g/mol. Most chemists use the terms interchangeably and nobody will correct you in the lab.

From Theory to Bench

Once you've got molar mass, pair it with the Molar Mass Calculator, validate experimental yield with the Percentage Error Calculator, and crunch any related arithmetic in the Scientific Calculator.

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