This molecular weight calculator sums atomic weights from a simple chemical formula. Enter a string like H2O to get MW = 2×1.008 + 15.999 = 18.015. That value is the molar mass style total from the tool’s element table.
Chemistry students use it before stoichiometry and solution prep. For decay over time, see the half life calculator. For bulk sample mass from density, use the mass calculator.
How the formula works
The parser reads each element symbol and optional count, looks up atomic weight, multiplies, and adds. Unsupported symbols or bad formatting return no valid total.
Worked example
Formula H2O. Hydrogen 1.008 × 2 = 2.016. Oxygen 15.999. Total molecular weight = 18.015.
| Input | Value |
|---|---|
| Formula | H2O |
| H contribution | 2.016 |
| O contribution | 15.999 |
| Molecular weight | 18.015 |
How to use the fields
- Formula is a compact alphanumeric string such as H2O, CO2, or NaCl.
Case sensitivity
Chemical symbols are case sensitive. Co is cobalt. CO is carbon monoxide. Typing CO as Co will misread the formula. Keep textbook capitalization.
Common mistakes
- Adding spaces or punctuation the parser does not allow
- Using parentheses for hydrates or complexes this simple helper does not expand
- Expecting isotope specific masses
- Confusing molecular weight with density
Lab checklist
- Write the formula with correct element case.
- Confirm each element is intended.
- Read the total molar mass.
- Use it to convert moles to grams in the next step of your lab math.
From molar mass to grams
Grams = moles × molecular weight. This page supplies the molecular weight piece. Mole counts still come from your balanced equation or desired preparation size.
More simple formulas
CO2 parses as C + O×2. NaCl parses as Na + Cl. Each total uses the tool’s atomic weight table. After you compute molecular weight, multiply by moles to get grams for a prep: grams = moles × MW.
Keep a lab notebook line that records the formula string exactly as typed so you can reproduce the same total later.
Stoichiometry bridge
Balanced equations speak in moles. Lab benches speak in grams. Molecular weight is the bridge. If a reaction needs 0.5 mol of water, grams ≈ 0.5 × 18.015 ≈ 9.0075 g using the default H2O total.
Do not mix molecular weight with density unless you are intentionally converting between mass and volume with an extra density step.
Parser discipline
Write H2O not h2o if you want standard element recognition. Write NaCl not NACL. Two letter elements need the second letter lowercase. Wrong case is the most common reason a formula fails to total.
Avoid spaces and punctuation. This helper is built for compact alphanumeric formulas, not full reaction equations.
Rounding and significant figures
Atomic weights in tables are already rounded. Your course may ask for a specific decimal policy on the final molar mass. Match the rubric. Small differences versus a textbook appendix usually come from table rounding, not from a different chemical identity.
For the default H2O result 18.015, keep the digits the tool reports unless your teacher requests a particular round off.
Percent composition teaser
After you know H2O is 18.015, hydrogen’s share is 2.016 ÷ 18.015 of the mass, and oxygen’s share is 15.999 ÷ 18.015. Percent composition problems start from that idea. This calculator stops at the total, which is the required first step.
Keep intermediate atomic contributions written down when a teacher asks for element percents, not only the final sum.
Prep worksheet flow
Write the target moles, look up or calculate MW, multiply for grams, then choose glassware. Skipping the MW line and guessing grams is how preps fail assay. For H2O, MW 18.015 keeps water preps boring in a good way: predictable and reproducible.
Store the MW next to the formula in your notebook template so every prep starts from the same constant.
Hydrates and parentheses note
Formulas like CuSO4·5H2O need expansion the simple parser may not accept as typed with dots or middle dots. Expand into a supported alphanumeric form only if you know the element counts, or compute pieces separately and add. Do not force unsupported punctuation into the field.
Limitations
Only simple formulas without nested parentheses are supported. Atomic weights follow the tool table and may differ slightly from a particular textbook appendix.