About

Accuracy in molecular weight calculation defines the success of stoichiometric planning and mass spectrometry analysis. Errors in manual summation, especially with complex organic chains or long peptide sequences, lead to wasted reagents and failed experiments. This tool addresses the specific needs of organic chemists and biochemists who require precise distinction between average molecular weight and monoisotopic mass.

Standard calculators often fail to handle batch processing or specific abbreviations common in organic synthesis. This implementation supports bulk input for high-throughput screening and includes a dedicated engine for peptide sequence analysis. By integrating isotopic distribution data, it serves as a reliable checkpoint for validating mass spec peaks against theoretical values.

Formulas

The calculation of molecular weight depends on the summation of atomic masses defined by the stoichiometry of the compound. For peptides, the calculation accounts for water loss during bond formation.

{

M_total = n∑i=1 (count_i × mass_i) General FormulaM_peptide = [k∑j=1 mass_residue,j] + mass_H2O Linear Peptide

Where count_i is the stoichiometric quantity of element i, and mass_residue represents the amino acid mass minus water (H₂O).

Reference Data

Entity	Formula / Symbol	Monoisotopic Mass (Da)	Average Mass (Da)
Carbon	C	12.00000	12.01070
Hydrogen	H	1.00783	1.00794
Nitrogen	N	14.00307	14.00670
Oxygen	O	15.99491	15.99940
Sulfur	S	31.97207	32.06500
Alanine (Residue)	Ala / A	71.03711	71.07880
Cysteine (Residue)	Cys / C	103.00919	103.13880
Phenylalanine (Residue)	Phe / F	147.06841	147.17660
Glycine (Residue)	Gly / G	57.02146	57.05190
Water	H₂O	18.01056	18.01528
Acetyl Group	Ac	43.01839	43.04500
Phenyl Group	Ph	77.03913	77.10600

Frequently Asked Questions

Average mass uses the weighted average of all natural isotopes (e.g., Carbon-12 and Carbon-13). Monoisotopic mass uses only the mass of the most abundant isotope (Carbon-12). As molecules get larger, the probability of containing heavier isotopes increases, causing the "Average" distribution to shift higher than the "Monoisotopic" peak used in high-resolution mass spectrometry.

By default, the peptide mode assumes free amine (N-term) and carboxyl (C-term) groups, effectively adding one water molecule to the sum of residues. If your peptide has modifications like Acetylation or Amidation, you should switch to Formula mode or manually add/subtract the specific modification mass.

In "Chemical Formula" mode, the tool parses specific case-sensitive abbreviations. "Ph" is expanded to C6H5, "Me" to CH3, and "Et" to C2H5. Ensure correct capitalization; "ph" or "PH" will not be recognized as the phenyl group.

Yes. You can input formulas using dot notation or standard grouping, such as "CuSO4(H2O)5". The parser treats parentheses as multipliers for everything contained within them.