About

Molecular weight errors propagate through every downstream calculation in chemistry: stoichiometry, molarity, yield prediction, and dosimetry. A single misidentified isotope or forgotten hydrate notation can shift a result by tens of grams per mole. This calculator parses chemical formulas using a recursive descent algorithm that correctly handles nested parentheses like Ca3(PO₄)₂, bracketed groups, coefficients, and hydrate notation (·). All atomic weights follow IUPAC 2021 standard values. For elements with no stable isotopes (e.g., Tc, Pm), the mass number of the longest-lived isotope is used, consistent with IUPAC convention.

The tool decomposes any valid formula into its elemental constituents with individual mass contributions and percentage composition. Note: this calculator uses standard atomic weights, not exact isotopic masses. For mass spectrometry or nuclear applications, isotope-specific data is required. Pro tip: always verify hydrate state of your reagents. Anhydrous CuSO₄ (159.61 g/mol) versus CuSO₄⋅5H₂O (249.69 g/mol) differ by 56%.

Formulas

The molecular weight M of a compound is the sum of atomic weights of all constituent atoms, each multiplied by their count in the formula:

M = n∑i=1 N_i ⋅ A_r,i

Where M = molecular weight in g/mol (numerically equal to unified atomic mass units, u). N_i = number of atoms of element i in the molecular formula. A_r,i = standard atomic weight of element i per IUPAC. n = number of distinct elements.

The mass fraction (percentage composition) of element i is computed as:

w_i = N_i ⋅ A_r,iM × 100%

For hydrates, the formula X⋅nH₂O is parsed as the sum of the anhydrous compound mass plus n times the mass of water (18.015 g/mol).

Reference Data

Z	Symbol	Element Name	Atomic Weight (u)	Group	Period
1	H	Hydrogen	1.008	1	1
2	He	Helium	4.0026	18	1
6	C	Carbon	12.011	14	2
7	N	Nitrogen	14.007	15	2
8	O	Oxygen	15.999	16	2
11	Na	Sodium	22.990	1	3
12	Mg	Magnesium	24.305	2	3
13	Al	Aluminium	26.982	13	3
14	Si	Silicon	28.086	14	3
15	P	Phosphorus	30.974	15	3
16	S	Sulfur	32.06	16	3
17	Cl	Chlorine	35.45	17	3
19	K	Potassium	39.098	1	4
20	Ca	Calcium	40.078	2	4
26	Fe	Iron	55.845	8	4
29	Cu	Copper	63.546	11	4
30	Zn	Zinc	65.38	12	4
35	Br	Bromine	79.904	17	4
47	Ag	Silver	107.868	11	5
53	I	Iodine	126.904	17	5
56	Ba	Barium	137.327	2	6
78	Pt	Platinum	195.084	10	6
79	Au	Gold	196.967	11	6
80	Hg	Mercury	200.592	12	6
82	Pb	Lead	207.2	14	6
92	U	Uranium	238.029	-	7

Frequently Asked Questions

Molecular weight and molar mass are numerically identical but differ in units: molecular weight is expressed in unified atomic mass units (u or Da), while molar mass uses g/mol. Formula weight is the preferred term for ionic compounds (e.g., NaCl) that do not exist as discrete molecules. This calculator outputs the value in both g/mol and u since the numeric value is the same.

The parser uses recursive descent. When it encounters an opening parenthesis or bracket, it enters a new scope, accumulates element counts within that scope, then multiplies all counts by the subscript following the closing delimiter. For Ca₃(PO₄)₂, it resolves the inner group to P×1 + O×4, multiplies by 2, then adds Ca×3. Nesting depth is limited to 10 levels to prevent abuse.

Elements with no stable isotopes (all radioactive) have no standard atomic weight defined by IUPAC. By convention, the mass number of the longest-lived or most commonly encountered isotope is listed in brackets. Examples: Tc [98], Pm [145], Np [237]. This calculator uses these conventional values.

Use the middle dot notation: type CuSO4·5H2O or CuSO4.5H2O (a period also works as a hydrate separator). The parser splits on · or . when followed by a digit and H2O pattern, sums the anhydrous mass plus n × 18.015 g/mol. Example: CuSO4·5H2O = 159.609 + 5 × 18.015 = 249.685 g/mol.

No. Standard atomic weights are weighted averages of naturally occurring isotopic abundances, which are properties of the element itself, not of thermodynamic state. Temperature and pressure affect density, molar volume, and gas behavior, but not atomic weight. However, in extreme environments (nuclear reactors, stellar interiors), isotopic ratios shift, making standard weights inapplicable.

Technically yes, up to the 500-character input limit. A hemoglobin subunit formula like C738H1166N812O203S2Fe would parse correctly. However, for macromolecules, rounding errors from standard atomic weights accumulate. For a 50 kDa protein, expect ±5 - 10 Da uncertainty versus exact isotopic computation.

The parser accepts standard chemical notation: element symbols (case-sensitive, e.g., Co for cobalt, CO for carbon monoxide), numeric subscripts directly after symbols or closing brackets, parentheses () and square brackets [] for grouping, and · or . for hydrate notation. Coefficients before a formula (e.g., 2H2O) are also supported. Spaces are ignored.