About Molar Mass & Molarity Calculator — Formula Weight & Solution Prep
The molar mass calculator turns a chemical formula into a formula weight in g/mol. Type the formula the way it is written — NaCl, H2SO4, Ca(OH)2, even a hydrate like CuSO4·5H2O (a plain * or . works as the dot) — and the parser expands parentheses and hydrate multipliers, sums each element's contribution from the IUPAC/CIAAW 2021 standard atomic weights, and shows the per-element breakdown so you can see exactly where the total comes from. Capitalization matters and the tool says so: CO is carbon monoxide, Co is cobalt, and an unknown symbol gets a clear error instead of a wrong number.
The second card answers the bench question that molar mass usually serves: how many grams do I weigh out? Solution preparation follows mass = molarity × volume × molar mass, so a 0.5 M solution of NaCl (58.44 g/mol) in a 2 L volumetric flask needs 0.5 × 2 × 58.44 = 58.44 g. The parsed formula feeds the prep calculation automatically. The atomic-weight table uses IUPAC's conventional values for the elements published as intervals (H 1.008, C 12.011, O 15.999, S 32.06, Cl 35.45, and so on) — the same two-to-three-decimal values printed on classroom periodic tables — which keeps formula weights accurate to a hundredth of a g/mol for everyday compounds.
How It Works
- Enter the chemical formula. Element symbols are one capital letter optionally followed by a lowercase letter; subscripts are plain digits (H2SO4); groups use parentheses with a trailing multiplier (Ca(OH)2, Ca3(PO4)2); hydrates append ·nH2O with a middle dot, asterisk, or period (CuSO4·5H2O).
- Read the formula weight in g/mol and the breakdown table: each element's atom count, its IUPAC/CIAAW 2021 atomic weight, and the subtotal — the columns sum to the headline number.
- For solution prep, enter the target molarity in mol/L and the final volume in liters. The tool multiplies through the parsed molar mass to give the mass to weigh out and the amount in moles.
- Weigh the solute, dissolve in less than the final volume, then fill to the mark — molarity is defined per liter of solution, not per liter of solvent added.
- For hydrates, weigh the hydrate's full formula weight: 0.1 mol of copper(II) sulfate as CuSO4·5H2O means 24.97 g, not the 15.96 g of the anhydrous salt — the five waters are part of the crystal you are putting on the balance.
Worked Example
Prepare 2 L of 0.5 M saline. Parsing NaCl: sodium contributes 1 × 22.99 and chlorine 1 × 35.45 (both IUPAC/CIAAW 2021 conventional values), so the formula weight is 58.44 g/mol. The prep step multiplies through: mass = 0.5 mol/L × 2 L × 58.44 g/mol = 58.44 g — dissolve that in distilled water and fill to the 2 L mark. The same workflow handles the awkward formulas: Ca(OH)2 expands the parenthesis to 2 O and 2 H around the calcium for 74.09 g/mol, and CuSO4·5H2O adds five waters (5 × 18.015) to the 159.60 g/mol anhydrous salt for 249.68 g/mol.
Molar mass of common compounds
Formula weights of six everyday compounds from the IUPAC/CIAAW 2021 atomic-weight table — type any formula into the tool to reproduce its row and see the per-element breakdown.
| Compound | Formula | Molar mass (g/mol) |
|---|---|---|
| Sodium chloride (table salt) | NaCl | 58.44 |
| Water | H2O | 18.02 |
| Sulfuric acid | H2SO4 | 98.07 |
| Sodium hydroxide (lye) | NaOH | 40.00 |
| Calcium carbonate (limestone) | CaCO3 | 100.09 |
| Glucose | C6H12O6 | 180.16 |
Formulas
- Molar mass (formula weight)
MW = Σ nᵢ · Aᵢ- Solution preparation
mass (g) = M (mol/L) × V (L) × MW (g/mol)- Hydrate expansion
CuSO4·5H2O → CuSO4 + 5 × H2O = 159.60 + 5 × 18.015 = 249.68 g/mol
Standards & References
- IUPAC/CIAAW, Standard Atomic Weights (2021 revision) — conventional values used for the 13 elements published as intervals (H 1.008, Li 6.94, B 10.81, C 12.011, N 14.007, O 15.999, Mg 24.305, Si 28.085, S 32.06, Cl 35.45, Ar 39.95, Br 79.904, Pb 207.2), abridged values elsewhere
- Molarity convention: mol of solute per liter of final solution (fill-to-the-mark), per IUPAC nomenclature
- 45-element table covering H through Bi — the elements that appear in common laboratory and industrial compounds
Frequently Asked Questions
What is the difference between molar mass, molecular weight, and formula weight?
Numerically they are the same thing in g/mol for a given formula. "Molecular weight" strictly applies to discrete molecules (H2O, C6H12O6), while "formula weight" covers ionic solids like NaCl whose crystal has no separate molecules — the value is per formula unit. "Molar mass" is the modern umbrella term: the mass of one mole (6.022 × 10²³ units) of whatever the formula describes. This calculator computes it identically for all three cases.
Why does capitalization matter in a formula?
Because element symbols are case-sensitive by definition: CO is one carbon and one oxygen (28.01 g/mol), while Co is cobalt (58.93 g/mol). Writing "co" or "CO" when you mean cobalt changes the parse completely. The parser reads one capital letter plus an optional lowercase letter as a symbol and rejects anything not in the table with the symbol named in the error, so a typo like Xx or a lowercase start fails loudly instead of returning a wrong mass.
How do I enter a hydrate like copper sulfate pentahydrate?
Type CuSO4·5H2O — and since the middle dot is awkward on most keyboards, CuSO4*5H2O and CuSO4.5H2O parse identically. The coefficient after the dot multiplies the whole water group: five waters add 5 × 18.015 = 90.08 g/mol to the 159.60 g/mol of anhydrous CuSO4, giving 249.68 g/mol. Weigh hydrates at the full hydrate mass — the crystal waters sit on your balance whether you want them or not.
How many grams do I need for a solution of given molarity?
mass = molarity × volume × molar mass. For 500 mL of 0.1 M NaOH: 0.1 × 0.5 × 40.00 = 2.00 g. Two classic mistakes: forgetting to convert mL to L (a factor-of-1000 error), and topping up to the final volume incorrectly — dissolve the solute in less water first, then fill to the mark, because molarity is per liter of finished solution, not per liter of water added.
Why do my textbook's atomic weights differ slightly from this tool's?
Since 2009 IUPAC publishes several elements as intervals (hydrogen is [1.00784, 1.00811]) because their isotopic composition varies by source. For everyday work IUPAC also issues a single "conventional" value per interval element — H 1.008, C 12.011, O 15.999 — and those are what this table uses, matching modern classroom periodic tables. Older books may carry pre-revision values (S 32.065, Cl 35.453); the differences land in the third decimal and rarely matter outside isotope work.
Can I use this for molality or percent solutions?
The parser's molar mass works for any concentration unit, but the prep card computes molarity (mol per liter of solution) specifically. For molality (mol per kg of solvent), multiply molality × solvent mass × MW instead. For a mass-percent solution, no molar mass is needed at all — it is grams of solute per 100 g of solution. If you have density data you can convert between them, but the fill-to-the-mark molar workflow here is the standard one for volumetric lab prep.