Molar Mass Calculator | Molecular Weight from Chemical Formula
Type any chemical formula and get the exact molar mass in g/mol instantly. The calculator handles parentheses, subscripts, and hydrates like CuSO₄·5H₂O. Atomic masses come from IUPAC 2025 standard weights, the same values NIST WebBook uses.
H2O, Ca(OH)2, or CuSO4·5H2O. Use a dot (·) or asterisk (*) for hydrates. Subscript numbers follow the element directly.Use element symbols (case-sensitive: Na not NA). Numbers follow directly: H2O not H₂O.
The Method That Never Fails
Molar mass tells you how many grams one mole of a substance weighs. Chemists calculate it by summing the atomic masses of every atom in the formula. The atomic masses come from IUPAC's 2025 table the same values this calculator uses.
Four-Step Method
Write the chemical formula and identify every element present.
Look up each element's standard atomic mass from IUPAC's table (or use this calculator).
Multiply each atomic mass by the number of atoms of that element in the formula.
Add all the products together — the total is the molar mass in g/mol.
Example 1: Water (H₂O)
Identify atoms: 2 hydrogen (H), 1 oxygen (O)
Atomic masses (IUPAC): H = 1.008 g/mol, O = 15.999 g/mol
Calculate: (2 × 1.008) + (1 × 15.999) = 2.016 + 15.999
✓ Molar mass of H₂O = 18.015 g/mol meaning 18.015 g of water contains exactly one mole of molecules
Example 2: Calcium Hydroxide Ca(OH)₂
Parentheses rule: The subscript 2 outside the parentheses multiplies every element inside.
Atoms: Ca × 1, O × 2, H × 2
Atomic masses: Ca = 40.078, O = 15.999, H = 1.008
Calculate: 40.078 + (2 × 15.999) + (2 × 1.008) = 40.078 + 31.998 + 2.016
✓ Molar mass of Ca(OH)₂ = 74.092 g/mol
Example 3: Hydrate CuSO₄·5H₂O
The dot (·) means 5 water molecules bond to each formula unit. Those water molecules are part of the mass you weigh on a balance.
CuSO₄ (anhydrous): Cu + S + 4O = 63.546 + 32.065 + 4(15.999) = 159.607 g/mol
5H₂O: 5 × 18.015 = 90.075 g/mol
✓ Total: 159.607 + 90.075 = 249.682 g/mol not 159.61 g/mol. Using the anhydrous value causes a 56% error in your solution concentration.
Why Molar Mass Errors Are Costly
Solution Preparation
Preparing 0.1 M NaCl in 1 L needs 5.844 g. Using 5.800 g (a wrong molar mass of 58.00) gives 0.0993 M a 0.7% systematic error that propagates into every measurement made from that solution.
Stoichiometry Reactions
Balanced equations work in moles, not grams. A wrong molar mass shifts every yield calculation. A student who uses 18.00 instead of 18.015 for water underestimates yield by 0.08% small, but systematic.
Gravimetric Analysis
Analytical chemists calculate analyte mass from precipitate mass using molar mass ratios. A 1% error in molar mass produces a 1% error in the reported concentration the kind that fails method validation.
Pharmaceutical Dosing
Drug assays convert UV absorbance to concentration using molar absorptivity, which requires accurate molar mass. The USP requires molar mass values to 4 significant figures for pharmacopeial calculations.
Common
Salt
Organic
Acid
Mineral
Base
Hydrocarbon
Alcohol
Fertilizer
Most common in lab formulas IUPAC 2025
Hydrates add mass
CuSO₄·5H₂O = 249.68 g/mol, not 159.61 g/mol. Check the bottle label for water of crystallization before weighing.
Element symbols are case-sensitive
Co = Cobalt. CO = Carbon + Oxygen. Na = Sodium. na will throw an error. The calculator reads standard IUPAC notation.
Grams to moles: divide by molar mass
25 g NaCl ÷ 58.44 g/mol = 0.428 mol. Moles to grams: multiply. 0.5 mol NaOH × 40.00 g/mol = 20.0 g.
Answers sourced from IUPAC 2025, NIST WebBook, and USP standards.
Every quantitative chemistry calculation starts with molar mass. It converts the practical world of grams what you weigh on a balance into the theoretical world of moles what chemical equations actually describe. This calculator parses any valid chemical formula and returns the molar mass in g/mol using IUPAC 2025 standard atomic weights.
Why Accurate Atomic Masses Matter
IUPAC updates standard atomic weights periodically as mass spectrometry techniques improve. The 2025 revision changed the standard atomic weight of hydrogen from 1.00794 to 1.008 and updated several other elements. These changes are small in absolute terms, but they affect high-precision analytical work. This calculator reflects the 2025 values.
In teaching labs, molar mass errors show up in two recurring patterns. First, students use the anhydrous molar mass for a hydrated salt CuSO₄ instead of CuSO₄·5H₂O because they overlook the label. Second, they round atomic masses aggressively, using 16 instead of 15.999 for oxygen. For a compound like K₂Cr₂O₇ (molar mass 294.18 g/mol), rounding each element to the nearest integer gives 294 g/mol close enough for a quick check, but a 0.06% systematic error in every gravimetric result.
Molar Mass in Analytical Chemistry
Gravimetric analysis converts the mass of a precipitate into the mass of analyte using a gravimetric factor a ratio of molar masses. Get either molar mass wrong and the factor shifts. A 0.1% error in the gravimetric factor creates a 0.1% error in every result derived from it, which matters when regulatory methods specify accuracy within ±0.2%.
Spectrophotometric methods use molar absorptivity (ε) in Beer-Lambert: A = ε × c × l. Converting absorbance to concentration requires accurate molar mass to convert g/L to mol/L. A wrong molar mass shifts all concentration results proportionally the calibration curve looks fine, but every interpolated value carries the error.
How This Calculator Works
The formula parser reads left to right, handling element symbols (one or two characters, case-sensitive), numeric subscripts, parentheses with their multipliers, and hydrate notation using a dot (·) or asterisk (*). Nested parentheses work correctly Ca₃(PO₄)₂ resolves to 3 calcium, 2 phosphorus, and 8 oxygen. The parser applies IUPAC 2025 atomic masses to each element count and sums the result.
Limitations
This calculator uses standard atomic weights, which are weighted averages of natural isotopic abundances. For isotopically enriched compounds or radiochemistry work, use the specific isotope mass instead. The calculator does not account for non-stoichiometric compounds, polymers with variable chain length, or materials where composition varies by batch. For those cases, use the empirically measured molecular weight from mass spectrometry.