Molecular Weight Calculator: Molar Mass and Weighing Mass from Chemical Formula
Molar mass is the first step in preparing a solution: how many grams to weigh depends on it. This page does two things — calculates molar mass from a chemical formula, then tells you exactly how much to weigh.
How it works
Molar mass M (g/mol) = Σ(atomic weight of each element × number of atoms of that element)
Mass to weigh m (g) = concentration c (mol/L) × volume V (L) × M (g/mol)
The second equation is the one you actually use at the bench. Note the unit conversions: 1 mM = 0.001 mol/L, 1 µM = 10⁻⁶ mol/L; volume 100 mL = 0.1 L.
How to write hydrates
Waters of crystallization must be included, or the moles you weigh out will be short. Three separator styles are accepted:
CuSO4·5H2O CuSO4*5H2O CuSO4.5H2O
This is the most common mistake in solution preparation: the bottle label shows the hydrate, but the literature often gives the anhydrous form. Anhydrous copper sulfate: 159.602; pentahydrate: 249.677 — a 56% difference. Using the anhydrous molecular weight to weigh out the pentahydrate gives a solution at only 64% of the target concentration.
Verification examples
| Formula | Molar mass (g/mol) | Reference |
|---|---|---|
NaCl |
58.44 | Textbook value |
H2O |
18.015 | Textbook value |
C6H12O6 |
180.156 | Glucose |
CuSO4·5H2O |
249.677 | Copper(II) sulfate pentahydrate |
C4H11NO3 |
121.136 | Tris base; matches 121.14 in this site’s buffer preparation calculator |
C10H14N2Na2O8·2H2O |
372.238 | Na₂EDTA·2H₂O; matches 372.24 in the same calculator |
Ca(OH)2 |
74.09 | Nested parentheses |
Fe2(SO4)3 |
399.858 | Nested parentheses + polyatomic group |
The last two rows are parser test cases: parentheses, subscripts, and nesting all have to work.
Which atomic weight values are used
The abridged standard atomic weights from CIAAW (the IUPAC Commission on Isotopic Abundances and Atomic Weights). The full table gives intervals rather than single values for more than a dozen elements — hydrogen is [1.00784, 1.00811], sulfur is [32.059, 32.076] — because natural isotope abundances genuinely vary with geological source. Bench-scale weighing does not require that precision, so this tool uses the abridged table’s single values, which are also the values on reagent bottle labels and in chemical handbooks.
Scope and limitations
- This calculates average molar mass (weighted by natural isotope abundances), for use in weighing. Mass spectrometry requires monoisotopic exact mass, which is a different number — use this site’s MS adduct m/z calculator for that.
- Charged species and isotope-labelling notation (
¹³C,SO4^2-) are not handled. For isotopically labelled compounds, correct the mass manually at the labelled positions. - The formula is not validated for chemical existence —
XeHe3will return a number. This is an arithmetic tool, not a chemical structure validator.
FAQ
Do waters of crystallization count toward the molecular weight?
Yes. If the label says CuSO₄·5H₂O, use 249.677, not the anhydrous 159.602 — a 56% difference. Using the anhydrous mass to weigh out the pentahydrate gives a final concentration of only 64% of target. This tool accepts `·`, `*`, and `.` as the hydrate separator. Watch for the reverse too: literature values are often for the anhydrous form, while the reagent on the shelf is a hydrate.
Is this the same molecular weight used in mass spectrometry?
No. This calculator returns the **average molar mass**, weighted by natural isotope abundances, which is what you use for weighing. Mass spectrometry uses the **monoisotopic exact mass** (taking only the most abundant isotope of each element); the two values differ by a few 0.0x Da. For caffeine C₈H₁₀N₄O₂, the average molecular weight is 194.19 and the monoisotopic mass is 194.0804. For the latter, use this site's MS adduct m/z calculator.
Why don't the atomic weights match the longer values in my textbook?
The full IUPAC standard atomic weights table gives **intervals** rather than single values for a dozen or so elements — hydrogen is [1.00784, 1.00811], sulfur is [32.059, 32.076] — because natural isotope abundances genuinely vary with geological source. This tool uses the CIAAW abridged table (single values), the same values used on reagent bottle labels and in chemical handbooks. For bench-scale weighing, this precision is more than adequate.
What if the calculated mass is too small to weigh on a balance?
Prepare a high-concentration stock solution and dilute. Below 1 mg the tool shows a warning — the reliable lower limit of a routine analytical balance is in the milligram range, and attempting to weigh microgram quantities introduces an error larger than the target concentration itself. For example, to make 500 mL of a 1 µM solution, first prepare a 10 mM stock and dilute 1:10000.
Which elements are supported?
The 84 elements that have standard atomic weights in the CIAAW abridged table, covering all common reagents. An unrecognised symbol triggers an explicit error identifying which symbol failed, rather than silently substituting 0 — which would return a plausible-looking but systematically low result.
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