How this instrument works
A mole is a fixed count of particles — 6.02214076×10²³ of them, Avogadro's constant — and it's the unit chemists use to talk about amounts of substance in a way that matches how reactions actually happen, atom by atom and molecule by molecule. A balance, though, measures mass, not particle count, so converting between the two requires knowing the molar mass: the mass of exactly one mole of a given substance, expressed in grams per mole (g/mol), which is numerically the same as the substance's atomic or molecular weight.
Molar mass is what makes the conversion substance-specific: one gram of hydrogen gas contains vastly more moles than one gram of lead, because a hydrogen molecule is dramatically lighter than a lead atom. Dividing a measured mass by the substance's own molar mass cancels out that difference and returns the mole count directly, which is the quantity a balanced chemical equation's coefficients actually refer to.
This conversion sits at the base of essentially every stoichiometric calculation — limiting reagent problems, theoretical yield calculations, solution concentration, and reaction balancing all start by getting a measured mass into moles first, since a balanced equation's coefficients describe mole ratios, not mass ratios. Get this step wrong and every downstream calculation inherits the error.
- Enter Mass (g) — the measured mass of your sample, in grams.
- Enter Molar mass (g/mol) — the substance's molar mass, found by summing its atomic weights from the periodic table (or looked up directly for a known compound).
- Read Moles (mol) directly — mass divided by molar mass.
- Molar mass must be greater than zero — every real substance has a positive molar mass, so a zero or negative entry has no chemical meaning and the instrument won't compute a result for one.
Worked example — 18 g of water
A sample of water weighs 18 g on the balance. Water's molar mass is 18.015 g/mol, from summing two hydrogen atoms (about 1.008 g/mol each) and one oxygen atom (about 15.999 g/mol). Enter 18 into Mass (g) and 18.015 into Molar mass (g/mol); Moles (mol) reads 0.999167360533.
That result is extremely close to, but not exactly, 1 mole — 18 g of water is only an approximation of one mole, since water's molar mass rounds to 18.015 g/mol rather than a perfectly clean 18. The commonly quoted rule of thumb that '18 g of water is about a mole' is accurate to roughly 0.08%, which is close enough for most classroom purposes but not mathematically exact.
Questions
Where do I find a substance's molar mass?
Add up the standard atomic weights, from the periodic table, of every atom in the substance's chemical formula. For water, H2O, that's two hydrogens (about 1.008 g/mol each) plus one oxygen (about 15.999 g/mol), totaling 18.015 g/mol. For an element on its own, the molar mass is simply that element's atomic weight as printed on the periodic table.
Why isn't 18 g of water exactly 1 mole?
Because water's actual molar mass, 18.015 g/mol, isn't a perfectly round 18 — it comes from summing hydrogen's and oxygen's real (non-integer) atomic weights. The commonly cited shortcut '18 grams of water is a mole' is a convenient approximation, accurate to a fraction of a percent, but the precise figure is 18/18.015 ≈ 0.9992 mol, very slightly under one full mole.
What's the difference between molar mass and molecular weight?
Numerically, they're the same value — a substance's molecular weight (or atomic weight for an element), a dimensionless number relative to the mass of carbon-12, and its molar mass in grams per mole (g/mol) share an identical number, just expressed with different units. This is a direct consequence of how the mole itself is defined, which is why chemists can look a molecular weight up on the periodic table and use that same number as a molar mass without any conversion factor in between.
Can this formula be used to convert moles back to grams?
Yes — rearranged, mass equals moles times molar mass, the same relationship run in the opposite direction. This instrument specifically solves for moles given a mass, but the identical molar-mass value works for the reverse conversion by hand: multiply your mole count by the molar mass to recover the mass in grams.
Once I have moles, how do I find the actual number of particles?
Multiply the mole count by Avogadro's constant, 6.02214076×10²³ per mole — this site's Avogadro's number calculator handles that second step directly. Moles and particle count are directly proportional to each other regardless of substance, unlike the mass-to-moles conversion, which depends on the specific substance's molar mass.