Molar Mass Calculator with Hydrates & Isotopes Support

Type your chemical formula exactly as you see it in your textbook. Simple compounds like H2O, NaCl, or CO2 work as expected. For compounds with parentheses, enter them directly: Ca(OH)2, Al2(SO4)3, or Mg(NO3)2. The parser handles nested groups too, so Ca3(PO4)2 calculates correctly as 3 calcium atoms plus 2 phosphate groups.

Hydrates need a dot or bullet between the salt and water. CuSO4·5H2O (copper sulfate pentahydrate) includes five water molecules in its crystal structure. Those waters add 90.08 g/mol to the anhydrous salt's mass. If your lab stock bottle says "CuSO4·5H2O," you must use the hydrated molar mass (249.69 g/mol), not the anhydrous value (159.61 g/mol). Grabbing the wrong number means your solution concentrations will be off by about 36%.

Watch capitalization. Co means cobalt; CO means carbon monoxide. Na is sodium, but NA won't parse. Element symbols always start uppercase, with the second letter lowercase if present. Numbers go immediately after the element they modify: H2SO4 has 2 hydrogens, 1 sulfur, and 4 oxygens.

This calculator uses standard atomic weights from IUPAC, the international body that maintains the periodic table. Carbon is 12.011 g/mol (not 12.000), because natural carbon contains about 1% carbon-13 alongside the dominant carbon-12. Chlorine averages 35.45 g/mol due to its 75:25 mix of Cl-35 and Cl-37 isotopes.

For most coursework, three decimal places are plenty. Hydrogen at 1.008, oxygen at 15.999, and nitrogen at 14.007 cover organic molecules well. Your final answer's precision should match your least precise measurement, not the atomic weight's precision. Weighing 2.5 g of NaCl on a centigram balance gives two sig figs regardless of how many decimals you use for sodium's atomic weight.

Some elements have variable atomic weights because their isotope ratios differ by source. Lithium mined from different locations can range from 6.938 to 6.997 g/mol. Sulfur varies between 32.059 and 32.076. For exam problems, use whatever value your instructor provides. For research, check the current IUPAC table and note any uncertainty ranges.

Standard molar mass uses natural abundance isotope mixtures. When you need a specific isotope, use caret notation: ^13C for carbon-13, ^2H or D for deuterium. Heavy water (D2O) has molar mass 20.03 g/mol, not 18.02 g/mol, because each hydrogen is replaced by deuterium at 2.014 amu.

Isotope-labeled compounds matter in NMR spectroscopy, metabolic tracer studies, and mass spec fragmentation analysis. If your synthesis uses ^13C-labeled glucose, the molecular ion shifts from m/z 180 to 186 (six carbons each gaining one mass unit). The calculator accepts mixed labeling: C5^13CH12O6 gives you glucose with one carbon-13 and five natural carbons.

Don't confuse isotopic mass with average atomic weight. Natural carbon at 12.011 represents the weighted average of C-12 and C-13. Pure carbon-13 has mass 13.003 amu. Using 12.011 for a fully ^13C-labeled compound gives wrong results by nearly 1 amu per carbon atom.

Every element contributes a fraction of the total molecular weight. For water: hydrogen accounts for 2(1.008)/18.015 = 11.19%, oxygen fills the remaining 88.81%. These percentages always sum to 100%, which provides a quick error check. If your composition adds to 97% or 103%, something went wrong in the calculation.

Percent composition connects formulas to experimental data. Combustion analysis of an unknown organic compound might give 40.0% C, 6.7% H, and 53.3% O. Convert percentages to moles: assume 100 g total, so you have 40.0 g C ÷ 12.01 = 3.33 mol C, and similarly for H and O. Divide each by the smallest mole value to get the ratio, then round to whole numbers for the empirical formula.

Empirical and molecular formulas can differ. Glucose (C6H12O6) and formaldehyde (CH2O) share the same 1:2:1 carbon-hydrogen-oxygen ratio, giving identical percent compositions. You need additional information—typically the molecular weight from mass spectrometry—to distinguish between them. Divide the measured molecular weight by the empirical formula weight to find the multiplier.

Parentheses distribution: Ca(OH)2 contains 2 oxygens and 2 hydrogens, not 1 of each. The subscript outside the parentheses multiplies everything inside. Al2(SO4)3 has 2 aluminums, 3 sulfurs, and 12 oxygens. Miss that multiplier and your mass is wrong by a factor related to the group size.

Atomic number confusion: Carbon's atomic number is 6, but its atomic weight is 12.011. The number at the top of the periodic table box counts protons only. The decimal number (atomic weight) includes neutrons and averages across isotopes. Using 6 instead of 12 for carbon halves your calculated mass.

Hydrate omission: Lab reagent bottles usually contain hydrated salts. Na2CO3·10H2O (washing soda) weighs 286 g/mol, not 106 g/mol. Ignoring those 10 waters means you're adding 2.7 times more sodium carbonate than intended. Always check the label for the dot and water count.

Rounding too early: Carry at least four decimal places through intermediate steps. Rounding carbon to 12.0 and oxygen to 16.0 might seem harmless, but cumulative errors grow in large molecules. C60H122 with premature rounding differs from the precise calculation by about 1 g/mol—enough to affect stoichiometry.

Problem: Calculate the molar mass of calcium phosphate, Ca3(PO4)2, and determine the mass percent of phosphorus.

Calcium phosphate contains about 20% phosphorus by mass. This matters for fertilizer calculations—farmers need to know how much actual phosphorus they're applying per acre, not just how much calcium phosphate compound they're spreading.