Free Molecular Weight Calculator — Every Step Shown

What molecular weight actually is

The molecular weight of a compound is the mass of one mole of it, in grams per mole. You get it by adding up the standard atomic mass of every atom in the formula. That is the whole calculation — the only skill involved is reading the formula correctly and not losing track of any atoms.

You will see three names for nearly the same number: molecular weight, molar mass, and formula mass. Molar mass is the modern, general term and always carries units of g/mol. "Formula mass" is preferred for ionic compounds like NaCl, which do not exist as discrete molecules — but the arithmetic is identical. Your teacher may insist on one name; the number does not change.

The atomic masses come from the IUPAC standard atomic weight table, which averages each element's naturally occurring isotopes. That is why carbon is 12.011 g/mol, not 12: about 1% of natural carbon is carbon-13. This calculator uses the current IUPAC values to four decimal places.

How to read a chemical formula

Most wrong answers come from misreading the formula, not from bad arithmetic. Four rules cover almost everything:

• A subscript counts only the symbol directly in front of it. In H2O, the 2 belongs to H alone: two hydrogens, one oxygen.
• A subscript after a closing bracket multiplies everything inside. Ca(OH)2 is one calcium, two oxygens, and two hydrogens — the 2 distributes over both the O and the H. Brackets nest: K4[Fe(CN)6] has one iron, six carbons, and six nitrogens inside the square brackets.
• The dot in a hydrate means "plus", not "times". CuSO4·5H2O is one CuSO4 unit plus five whole water molecules: add 5 × 18.015 to the mass of CuSO4. Type the dot as ·, a period, or an asterisk — all three work here.
• Capitalization is meaning, not style. Co is cobalt (58.93 g/mol); CO is carbon monoxide (28.01 g/mol). This calculator never changes your capitalization, and it will tell you exactly which symbol it could not read rather than guessing.

Worked example: CuSO4·5H2O

Copper(II) sulfate pentahydrate is the classic exam formula because it tests brackets-free reading, the hydrate dot, and careful addition all at once. Here is the full working, the same way the calculator lays it out.

Step 1 — read the formula. There are two parts: CuSO4 (the main unit) and 5H2O (five water molecules attached to the crystal).

Step 2 — look up the atomic masses: Cu = 63.5460, S = 32.0600, O = 15.9990, H = 1.0080 g/mol.

Step 3 — multiply and add, one part at a time. CuSO4: (1 × 63.5460) + (1 × 32.0600) + (4 × 15.9990) = 63.5460 + 32.0600 + 63.9960 = 159.6020 g/mol. Water: (2 × 1.0080) + 15.9990 = 18.0150 g/mol, and five of them is 5 × 18.0150 = 90.0750 g/mol.

Step 4 — add the parts: 159.6020 + 90.0750 = 249.6770 g/mol, which you would report as 249.68 g/mol. If your textbook says 249.69, it is using an older atomic-weight table — see the FAQ; both are acceptable answers.

Once you have the molar mass, grams-to-moles is one division. Dissolving 25.0 g of CuSO4·5H2O: n = 25.0 g ÷ 249.677 g/mol = 0.100 mol.

The mistakes that actually cost marks

• Forgetting to distribute a bracket subscript. Al2(SO4)3 has three entire sulfate groups: 3 sulfurs and 12 oxygens, not 3 sulfurs and 4 oxygens. Correct answer: 342.13 g/mol.
• Multiplying instead of adding at the hydrate dot. The ·5H2O in CuSO4·5H2O adds 90.075 g/mol; it does not multiply anything.
• Dropping the water entirely. If a problem gives you the pentahydrate, its molar mass is 249.68, not 159.60. Using the anhydrous mass makes every downstream mole calculation wrong by 36%.
• Case errors. Typing CO when you mean Co, or NA when you mean Na. A calculator that "helpfully" fixes capitalization can silently compute a different substance — this one refuses and tells you why.
• Ambiguous ion notation. Mg2+ could mean Mg²⁺ (one ion, 24.31 g/mol) or Mg₂⁺ (two atoms, 48.61 g/mol). This calculator rejects it and asks you to write Mg^2+ — the charge itself never changes the mass, but which digits are atom counts does.
• Rounding too early. Round once, at the end. If you round each atomic mass to whole numbers first, K4[Fe(CN)6] comes out at 368 instead of 368.35 — enough to lose a mark in a multi-step problem.

How to check your answer

Three quick checks catch nearly every slip:

• Count atoms before you add masses. Write out the tally (CuSO4·5H2O → Cu 1, S 1, O 9, H 10) and confirm it against the formula. The calculator's breakdown table is exactly this tally — if its counts differ from yours, you misread a bracket or the hydrate dot.
• Use percent composition as a sanity check. The percentages must add to 100, and the biggest contributor should make chemical sense: in CuSO4·5H2O, oxygen is 57.7% of the mass because there are nine of them. If an element's percentage looks absurd, its count is wrong.
• Compare against anchor values you know: H2O = 18.02, CO2 = 44.01, NaCl = 58.44, glucose C6H12O6 = 180.16. If your method reproduces those, your method is sound — any remaining disagreement is a lookup or copying error, not a concept error.