Free Molar Mass Calculator – Molar Mass and Mass Percent from Any Formula

What Molar Mass Is and How the Composition Table Is Built

Molar mass is the mass of one mole of a substance, written in grams per mole. It is the number you need before any weighing-out step, because a balance reads grams while a reaction equation counts moles, and molar mass is the bridge between the two. Type a formula such as CuSO₄·5H₂O and this tool returns 249.677 g/mol together with a per-element table showing exactly how much of that total each element contributes. Students reach for it during stoichiometry homework, lab technicians use it when preparing a standard solution, and percentage-composition questions need the same breakdown. Molecular weight is the identical figure expressed in unified atomic mass units, so 249.677 u and 249.677 g/mol describe the same compound. The arithmetic is a weighted sum, M = Σ (nᵢ × Aᵢ). Here M is the molar mass in g/mol, nᵢ is the count of atoms of element i in one formula unit, and Aᵢ is that element's standard atomic weight in g/mol from the IUPAC 2021 table. Mass percent for each element is (nᵢ × Aᵢ) ÷ M × 100, and the unrounded column sums to exactly 100. Moles follow from n = m ÷ M with m in grams, and the particle count is n × Nᴀ, where Nᴀ is 6.02214076 × 10²³ mol⁻¹, exact by definition since the 2019 redefinition of the mole. A recursive-descent parser reads the formula, handling nested brackets, hydrate dots, multi-digit subscripts and leading coefficients. Because an uppercase letter starts a symbol and lowercase letters continue it, an unrecognised token raises a named error instead of a silent guess. Follow copper(II) sulfate pentahydrate through. The formula CuSO₄·5H₂O contains one copper, one sulfur, nine oxygens and ten hydrogens once the five water molecules are counted in. Copper contributes 63.546, sulfur 32.060, oxygen 9 × 15.999 = 143.991 and hydrogen 10 × 1.008 = 10.080, summing to 249.677 g/mol. Divide each subtotal by that total and the composition reads oxygen 57.671%, copper 25.451%, sulfur 12.841% and hydrogen 4.037%. Weigh out 10 g of the crystals and you have 10 ÷ 249.677 = 0.040052 mol, which is 2.4120 × 10²² formula units. Other check values: H₂O is 18.015, CaCO₃ is 100.086, glucose C₆H₁₂O₆ is 180.156, NaCl is 58.4398 and sucrose C₁₂H₂₂O₁₁ is 342.297 g/mol. Three situations make the composition table matter more than the total. A student answering "what percentage of calcium carbonate is oxygen?" needs 47.956%, which comes from 47.997 ÷ 100.086 — the total alone does not answer the question. A technician preparing 250 mL of 0.15 M glucose needs 6.7558 g, obtained by multiplying 0.15 mol/L × 0.25 L × 180.156 g/mol, so an error in the molar mass propagates straight into the solution strength. And anyone working with hydrates has to decide whether the water counts: anhydrous CuSO₄ is 159.609 g/mol against 249.677 for the pentahydrate, a 56% difference that quietly ruins a preparation if the bottle label is read carelessly. One pitfall deserves naming, because it is invisible once typed. Co is cobalt at 58.933 g/mol; CO is carbon monoxide at 28.010 g/mol. A single shift key changes the answer by 31 g/mol, and the same trap catches No against NO and Sn against SN. This parser follows chemical notation strictly and warns when an all-caps pair could have been meant as one element. Atomic weights come from the IUPAC 2021 Table of Standard Atomic Weights, so an older calculator can differ in the third decimal — sulfur moved from 32.065 to 32.06, shifting Fe₂(SO₄)₃ by about 0.02 g/mol. Standard atomic weights are terrestrial isotopic averages, so they do not describe enriched material such as heavy water. Everything is parsed in your browser and nothing you type is uploaded.

Molar Mass Calculator Formula & Method

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