# Molar Mass API > Molar-mass and stoichiometry maths as an API, computed locally and deterministically. The molarmass endpoint parses any chemical formula — with parentheses, square brackets and hydrate dots, such as Ca(OH)2, [Fe(CN)6]3 or CuSO4·5H2O — against the IUPAC conventional atomic weights and returns the molar mass in grams per mole, the total atom count and the per-element breakdown with each element's mass contribution and mass percent. The convert endpoint moves between moles, mass in grams and number of molecules for a formula, using n = mass ÷ M = molecules ÷ Nₐ with Avogadro's number. The percent endpoint gives the percent composition by mass and, for a given sample mass, the mass of each element it contains. The formula is parsed locally, so it works for any valid formula, not just compounds in a database, and is instant and private. Ideal for chemistry-education, laboratory, pharmaceutical and science app developers, stoichiometry and lab-prep tools, and STEM teaching. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This computes molar mass from a formula; for compound database lookup use a chemistry API and for element properties an elements API. ## Authentication All requests require your oanor API key in the `x-oanor-key` header. Get one at https://www.oanor.com/developer/keys. ```bash curl -H "x-oanor-key: oanor_live_…" "https://api.oanor.com/molarmass-api/..." ``` ## Pricing - **Free** (Free) — 3,000 calls/Mo, 2 req/s - **Starter** ($5/Mo) — 40,000 calls/Mo, 5 req/s - **Pro** ($15/Mo) — 250,000 calls/Mo, 15 req/s - **Mega** ($45/Mo) — 1,500,000 calls/Mo, 40 req/s ## Endpoints ### Molar Mass #### `GET /v1/convert` — Mole / mass / molecules **Parameters:** - `formula` (query, required, string) — Chemical formula Example: `H2O` - `moles` (query, optional, string) — Amount (mol) Example: `2` - `mass` (query, optional, string) — Or mass (g) - `molecules` (query, optional, string) — Or number of molecules **Example:** ```bash curl -H "x-oanor-key: $KEY" \ "https://api.oanor.com/molarmass-api/v1/convert?formula=H2O&moles=2" ``` **Response:** ```json { "data": { "note": "n = mass/M = molecules/Nₐ. Nₐ = 6.02214076×10²³ mol⁻¹.", "moles": 2, "inputs": { "formula": "H2O", "molar_mass_g_mol": 18.015 }, "mass_g": 36.03, "molecules": 1.204428152e+24 }, "meta": { "timestamp": "2026-06-04T18:38:04.792Z", "request_id": "72625459-77e7-4358-9adc-3fa26c0557ac" }, "status": "ok", "message": "Mole convert", "success": true } ``` #### `GET /v1/molarmass` — Molar mass of a formula **Parameters:** - `formula` (query, required, string) — Chemical formula Example: `CuSO4.5H2O` **Example:** ```bash curl -H "x-oanor-key: $KEY" \ "https://api.oanor.com/molarmass-api/v1/molarmass?formula=CuSO4.5H2O" ``` **Response:** ```json { "data": { "note": "Molar mass = Σ(atomic weight × count), using IUPAC conventional atomic weights. Percent is the mass fraction of each element.", "inputs": { "formula": "CuSO4.5H2O" }, "elements": [ { "count": 1, "element": "Cu", "percent": 25.451283, "atomic_weight": 63.546, "mass_contribution": 63.546 }, { "count": 1, "element": "S", "percent": 12.84059, "atomic_weight": 32.06, "mass_contribution": 32.06 }, { "count": 9, "element": "O", "percent": 57.670911, "atomic_weight": 15.999, "mass_contribution": 143.991 }, { "count": 10, "element": "H", "percent": 4.037216, "atomic_weight": 1.008, "mass_contribution": 10.08 } ], "atom_count": 21, "molar_mass_g_mol": 249.677 }, "meta": { "timestamp": "2026-06-04T18:38:04.898Z", "request_id": "19ee7c8a-4f3e-40b7-96eb-ba5776846a07" }, "status": "ok", "message": "Molar mass", "success": true } ``` #### `GET /v1/percent` — Percent composition **Parameters:** - `formula` (query, required, string) — Chemical formula Example: `C6H12O6` - `mass` (query, optional, string) — Sample mass (g) for element masses Example: `100` **Example:** ```bash curl -H "x-oanor-key: $KEY" \ "https://api.oanor.com/molarmass-api/v1/percent?formula=C6H12O6&mass=100" ``` **Response:** ```json { "data": { "note": "Mass percent = (element mass in one mole / molar mass) × 100. With a sample mass, the mass of each element is given too.", "inputs": { "formula": "C6H12O6", "sample_mass_g": 100, "molar_mass_g_mol": 180.156 }, "composition": [ { "mass_g": 40.001998, "element": "C", "percent": 40.001998 }, { "mass_g": 6.714181, "element": "H", "percent": 6.714181 }, { "mass_g": 53.283821, "element": "O", "percent": 53.283821 } ] }, "meta": { "timestamp": "2026-06-04T18:38:05.015Z", "request_id": "e9bd47a0-c1af-4f3c-b8ff-e914e0374739" }, "status": "ok", "message": "Percent comp", "success": true } ``` ### Meta #### `GET /v1/meta` — Spec **Example:** ```bash curl -H "x-oanor-key: $KEY" \ "https://api.oanor.com/molarmass-api/v1/meta" ``` **Response:** ```json { "data": { "notes": "Formulas support parentheses, square brackets and hydrate dots (·, ., *), e.g. Ca(OH)2, [Fe(CN)6]3, CuSO4.5H2O. IUPAC conventional atomic weights; Nₐ = 6.02214076×10²³.", "service": "molarmass-api", "formulae": { "mole": "n = mass / M = molecules / Nₐ", "percent": "element mass / molar mass × 100", "molar_mass": "Σ(atomic weight × count)" }, "endpoints": { "GET /v1/meta": "This document.", "GET /v1/convert": "Convert between moles, mass (g) and molecules for a formula.", "GET /v1/percent": "Percent composition, and the mass of each element in a given sample mass.", "GET /v1/molarmass": "Molar mass, atom count and per-element percent composition of a formula." }, "description": "Molar-mass and stoichiometry calculator: parse a chemical formula (parentheses, brackets, hydrates) for its molar mass and percent composition, and convert between moles, mass and molecules." }, "meta": { "timestamp": "2026-06-04T18:38:05.142Z", "request_id": "b94d3843-0e6b-42b6-b52a-d77030111f09" }, "status": "ok", "message": "Meta", "success": true } ``` --- Marketplace page: https://www.oanor.com/api/molarmass-api OpenAPI spec: https://www.oanor.com/api/molarmass-api/openapi.json