How do I convert grams to moles?

Divide grams by molar mass: moles = grams / M. For example, 36 g of water (M = 18.015 g/mol) = 36 / 18.015 = 1.998 mol. The molar mass for any substance is the sum of atomic masses of every atom in the formula, in g/mol units.

Molar mass (M, units g/mol) is the mass of one mole of a substance. Numerically it equals the molecular weight (M_r, dimensionless) but with units g/mol. For H₂O: 2(1.008) + 16.00 = 18.015 g/mol. IUPAC publishes atomic weights every two years; 2024 values used here are NIST-traceable.

Why does the periodic table give weird atomic masses like 35.453 for chlorine?

Atomic weight is a weighted average over natural isotopic abundance. Chlorine is 75.78% ³⁵Cl (34.969 u) and 24.22% ³⁷Cl (36.966 u) - the weighted mean is 35.453 u. For isotopically pure samples or radio-isotopes, use the integer mass number instead.

How precise are the molar masses in this calculator?

We use 2024 IUPAC atomic-weight recommendations to four decimal places where available. For elements with large isotopic variation (Li, B, Mg, Pb) IUPAC publishes interval values - we use the recommended single value. Reproducibility uncertainty is typically ±0.0005 g/mol.

What is the difference between molecular mass and formula mass?

Molecular mass applies to discrete molecules (H₂O, CO₂). Formula mass applies to ionic compounds without distinct molecules (NaCl, CaCO₃) - it is the mass of one formula unit. Both are numerically equal to the molar mass in g/mol. The mole counts entities; what an 'entity' is depends on the substance.

Can I use this for hydrated salts?

Yes - include the water of hydration in the molar mass. Copper(II) sulfate pentahydrate CuSO₄·5H₂O has M = 159.609 (anhydrous) + 5 × 18.015 = 249.685 g/mol. Use the molar mass calculator for arbitrary formulas including the dot notation.

Why is 1 mole of H₂ only 2 grams but 1 mole of Fe is 56 grams?

Mass per atom varies enormously across the periodic table. Hydrogen-1 has a mass of 1.008 Da; iron-56 has a mass of 55.847 Da - about 55× heavier. A mole counts the same number of entities (6.022 × 10²³) regardless, so the gram-equivalent scales with the per-atom mass.

How do I weigh a substance accurately enough to compute moles?

Use an analytical balance (0.1 mg resolution = 10⁻⁴ g) for small samples, or a top-loader (10 mg) for bulk. Tare the weighing boat first, dispense, and read at thermal equilibrium. A 1.000 g sample on a 0.1 mg balance gives 4 significant figures, which propagates to 4-sig-fig moles.

What is the gram-formula-weight (GFW) on a chemical reagent bottle?

GFW is the supplier's term for molar mass in g/mol. The Sigma-Aldrich, Fisher, and Merck catalogues print GFW on every bottle label. It is identical to M for the formula listed - use the catalogue value rather than recomputing if your supplier specifies an unusual hydration state.

Why do I get slightly different moles depending on which atomic-weight source I use?

Older textbooks may use 1990s atomic weights (carbon was 12.011 then, 12.011 now - unchanged) but lighter elements have been revised. CIAAW publishes the canonical values; for analytical work below 0.1% accuracy this rarely matters. For metrology, use the current CODATA / IUPAC release.

How was the kilogram redefinition in 2019 related to the mole?

Before 20 May 2019 the kilogram was defined by the IPK artefact and the mole was defined by carbon-12. After 2019 the kilogram is defined by fixing Planck's constant h, and the mole by fixing Avogadro's number Nₐ. The two are now mutually independent base units rather than interdependent.

Can I compute moles from grams of a mixture?

Only if you know the composition. For pure substances the math is direct. For mixtures (e.g., 70% nitric acid), multiply by mass fraction first: 100 g of 70% HNO₃ = 70 g HNO₃ = 70 / 63.012 = 1.111 mol. Stock-acid concentration tables (Merck Index) tabulate this for common reagents.

IUPAC 2024 atomic weights · 20 preset substances · Analytical-grade math

Grams to Moles Calculator

To convert grams to moles, divide the mass in grams by the substance's molar mass: n = m / M. Pick from 20 named substances or enter a custom molar mass; the live balance dial visualises both quantities. NIST-traceable molar masses, IUPAC 2024 atomic weights.

Presets

20 substances

Atomic weights

IUPAC 2024

Formula

n = m / M

Traceable

NIST

Quick Conversion

FROM (g)

TO (mol)1

Molar mass M (g/mol)

Formula: mol = grams / M

Analytical Balance + Molar Mass Dial

Sample mass tilts the beam; the dial confirms M.

Mass in gramsSubstance preset

Grams

18.0150

Moles

1.0000

Particles

6.02e+23

What this answer really means

Weighing out 18.0150 g of Water delivers 1.0000 moles — or 6.0221e+23 molecules. Universal solvent. 18.015 g/mol exact by 2019 atomic masses.

20-substance reagent library

Grams → Moles for water (M = 18.015 g/mol)

Grams	Moles H₂O	Molecules
0.018	0.00100	6.02e+20
0.18	0.00999	6.02e+21
1	0.05551	3.34e+22
2	0.11102	6.69e+22
5	0.27755	1.67e+23
9	0.49958	3.01e+23
18.015	1.00000	6.02e+23
36.03	2.00000	1.20e+24
90	4.99584	3.01e+24
180	9.99167	6.02e+24
500	27.75465	1.67e+25
1000	55.50930	3.34e+25
5000	277.54649	1.67e+26

Need to go the other way? Compute moles → grams using molar mass

Formula

n (mol) = m (g) / M (g/mol)

Worked example: Weigh 5.85 g of NaCl. M(NaCl) = 22.990 + 35.453 = 58.443 g/mol. → n = 5.85 / 58.443 = 0.1001 mol. That is 0.1001 × 6.022 × 10²³ = 6.03 × 10²² formula units in the weighing boat.

How to weigh and convert in 5 steps

Identify the substance. Pick from the 20-preset library (water, NaCl, glucose, NaOH...) or enter a custom formula via the molar-mass calculator.
Tare and weigh. On an analytical balance, tare the weighing boat first, dispense your sample, and read the mass at thermal equilibrium for 4-significant-figure precision.
Apply n = m / M. The calculator divides mass by molar mass instantly. The balance widget tilts proportionally so you can sanity-check the magnitude visually.
Check the particle count. Cross-multiply by Avogadro's number (6.022 × 10²³) to get the number of molecules / formula units / atoms in your sample.
Save the record. Press Save snapshot to log substance + mass + moles into browser-local history (20-entry rolling buffer) for downstream lab-notebook export.

How chemistry learned to weigh atoms

Why this calculator exists: In 2026, a pharma QC scientist at a CDMO must document a grams-to-moles conversion for every reagent that enters a batch record. The IUPAC-compliant molar masses, the substance preset list, and the auditable snapshot history collapse what used to be a balance + a calculator + a periodic-table chart into one screen.

The conceptual leap that made 'grams to moles' possible was Cannizzaro's 1860 talk at the Karlsruhe Congress. Until then chemists could not agree whether water was HO (so O = 8) or H₂O (so O = 16). Cannizzaro showed how Avogadro's gas-volume hypothesis fixed the formula and therefore the relative atomic weights. Within a decade Mendeleev had used those weights to publish the periodic table (1869), and stoichiometric weighing - the heart of n = m/M - became a routine bench skill.

The periodic table itself encodes a grams-to-moles instruction. Every cell prints an atomic weight in atomic-mass units (u or Da) that numerically equals the molar mass in g/mol. When Mendeleev arranged elements by atomic weight in 1869, he was creating the world's first molar-mass lookup table. Modern IUPAC tables (2024 release) revise weights periodically as isotope-ratio measurements improve - chlorine moved from 35.453 to the interval [35.446, 35.457] in 2009, settling at 35.45 today.

Twentieth-century metrology made the gram exquisitely reproducible. The kilogram was defined by Le Grand K, a platinum-iridium artefact at the BIPM in Paris from 1889 to 2019. National prototypes (NIST K20 in the US, PTB K58 in Germany) were periodically compared. Small drifts of micrograms accumulated, which prompted the 2019 redefinition - the kilogram is now realised from the Planck constant via Kibble balances. A gram is one-thousandth of that, and the molar mass of every substance changed by less than the experimental precision of any bench balance.

Joseph Loschmidt (1865) first estimated Avogadro's number from gas kinetic theory; Jean Perrin (1909) refined it with Brownian motion and won the 1926 Nobel Prize. Ernest Rutherford's 1911 nuclear model gave atoms a structure to weigh: the nucleus dominates the mass. Henry Moseley's 1913 X-ray spectra established atomic number Z as the periodic-table organiser, finishing what Mendeleev had started 44 years earlier. By the time chemists were doing routine n = m/M on the bench in the 1920s, every step in the chain - atomic weight → molar mass → grams → moles - had a primary-source paper behind it.

The 20 May 2019 SI redefinition completed the metrological story. The kilogram is fixed by h, the mole by Nₐ, and the molar mass of carbon-12 is now an experimentally measured quantity (≈ 12 g/mol, with finite uncertainty around the eighth decimal). For practical chemistry the change is invisible - 18.015 g/mol for water is still 18.015 g/mol - but the constants are now linked to fundamental physics rather than to artefacts in a Paris vault.

Today this calculator implements n = m/M with NIST-traceable molar masses, lets you swap among 20 reagents instantly, and visualises the balance + dial that bench chemists have been using for 200 years. For counting-based conversions see atoms to moles; to derive M from a formula see the molar mass tool.

Grams ↔ moles - frequently asked questions

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