Molecular Weight Calculator

Related Calculators

Chemical formula: A short way to show which elements are in a substance and how many atoms of each are present.

Molar mass: The mass of 1 mole of a substance, shown in g/mol. This is usually the main chemistry-class answer.

Formula mass: The mass of one molecule or one formula unit, shown in u. Its number matches the molar mass number for the same formula, but the unit is different.

Parsed formula: The calculator's cleaned-up reading of your entry after it applies group counts, isotopes, and hydrate parts.

Element counts: The total number of each kind of atom in one formula unit.

Mass percent by element: How much each element contributes to the total mass, written as a percent.

Known value: In conversion mode, this tells the calculator whether you are starting with mass or moles.

Isotope notation: A way to name a specific version of an element by mass number, such as [13C] for carbon-13.

Mistake: Typing an invalid symbol in Chemical formula , such as COa instead of CoO.
Fix: Check capitalization carefully. Element symbols are case-sensitive, so Co is cobalt but CO means carbon plus oxygen.

Mistake: Forgetting a closing parenthesis in Chemical formula , such as Ca(OH2.
Fix: Make sure every opening parenthesis or bracket has a matching closing one before you calculate.

Mistake: Turning on conversion in Result type but leaving Known mass (g) or Known amount (mol) blank.
Fix: After choosing conversion, fill in the visible required field that matches your Known value selection.

Mistake: Entering a negative number in Known mass (g) or Known amount (mol) .
Fix: Use a positive value only. Sample mass and amount cannot be negative in this calculator.

Mistake: Reading Formula mass and Molar mass as different numbers that should not match.
Fix: The numeric value is the same for the same formula here, but the units are different: u for Formula mass and g/mol for Molar mass .

Mistake: Ignoring Parsed formula and Element counts after a complicated entry like CuSO4*5H2O or [13C]6H12O6.
Fix: Use those outputs to confirm the calculator read hydrate parts, isotopes, and group counts the way you intended.

• The calculator depends on a valid formula. Unknown element symbols, missing brackets, missing parentheses, or zero or negative atom counts will cause an error instead of a result.
• Natural-abundance calculations use abridged standard atomic weights [1]. Real lab values can differ slightly because of rounding or isotope composition.
• If you enter an explicit isotope such as [13C], that atom is treated with the stated isotope mass choice rather than the usual natural-abundance atomic weight.
• For radioactive elements, a standard atomic weight may not exist [4]. In those cases, the notes output may tell you to use isotope-specific entry.
• Mass percent values may not add to exactly 100.00% after rounding. Small differences such as 99.99% or 100.01% are rounding effects, not necessarily a formula mistake.
• Conversion only works after a valid formula produces a positive molar mass. Blank, negative, or invalid values in Known mass (g) or Known amount (mol) are blocked.

How the calculator finds the answer

The calculator first reads the Chemical formula from left to right. It identifies element symbols, counts after symbols, grouped atoms in parentheses, isotope tags in square brackets, and hydrate parts joined by * or . Then it totals how many atoms of each element appear in one full formula unit.

M = sum(n_i * A_i)

The molar mass M is found by multiplying each element count by its atomic weight, then adding all contributions. The calculator uses abridged standard atomic weights for natural-abundance entries [1].

m_u = sum(n_i * A_i)

The formula mass in u uses the same numeric total, but the unit means mass of one molecule or one formula unit instead of grams per mole. The unit u is defined from carbon-12 [4].

Mass percent and conversion

mass_percent_i = (n_i * A_i / M) * 100

This gives each element's share of the total mass. It helps you check whether heavy elements or repeated groups are affecting the answer the way you expected.

n = m / M

m = n * M

If you choose conversion mode, the calculator uses the molar mass to change grams into moles or moles into grams.

Mini example

For H2O, the parsed counts are H = 2 and O = 1. Using student-friendly atomic weights, hydrogen contributes about 2 x 1.008 = 2.016 and oxygen contributes about 1 x 15.999 = 15.999. Adding them gives about 18.015, so the calculator reports Molar mass = 18.015 g/mol and Formula mass = 18.015 u.

If Result type is conversion and Known value is mass with Known mass (g) = 18, then the amount is about 18 / 18.015 = 0.9992 mol.

How invalid formulas are handled

If the parser finds a bad token, unmatched bracket, unmatched parenthesis, or impossible count, it stops and shows a specific error so you can fix the exact part of the formula. It also ignores hidden conversion fields so formula-only mode does not accidentally use unused values.

Assumptions used

The calculator assumes your formula describes one complete formula unit and that counts are positive whole-number multipliers. It supports common classroom notation, including one or more grouped parts, hydrate notation, and square-bracket isotope tags. Results can differ slightly from other tools if they use different rounding, different atomic-weight tables, or different isotope assumptions [3][3].