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Grams to Moles Converter

Chemistry

Substance

Symbols are case-sensitive. Supports ( ), [ ], hydrates (CuSO4·5H2O) and charges (SO4^2-, NH4+, Fe³⁺).

Interpreted as H₂O · water · M = 18.015 g/mol

Common substances

Quantity

Type in either field. The one you typed in last is the input; the other is calculated.

Precision

Amount of substance

0.555 mol H₂O

= 555 mmol

Rounded to 3 s.f. (from 10.0 g; molar mass known to 5 s.f.)

Molar mass
18.015 g/mol
Particles
3.34 × 10²³
molecules / formula units
Total atoms
1.00 × 10²⁴
3 per formula unit
Mass of one particle
2.991 × 10⁻²³ g
M / N_A, 4 s.f.

Factor-label working

Each unit that appears on top and bottom cancels. Matching pairs share a colour and a strike line.

= 10.0 ÷ 18.015 = 0.55509… → 0.555 mol (3 s.f.)

Where the sample's mass comes from

Each element's share of the sample, largest first.

O 8.88 g (88.8 %)
H 1.12 g (11.2 %)
ElementAtoms per unitMass %Mass (g)Moles of atoms (mol)Number of atoms
OOxygen
188.818.880.5553.34 × 10²³
HHydrogen
211.191.121.116.69 × 10²³
Total3100.0010.01.671.00 × 10²⁴

About This Tool

Grams to Moles Converter – Mass, Moles and Particles in One Step

Chemists weigh substances in grams but count them in moles. Every stoichiometry problem, every reagent you weigh out and every yield calculation starts by converting between the two. This grams to moles converter reads the chemical formula you type, works out its molar mass from standard atomic weights, and converts a mass into an amount in moles, or an amount back into grams. It also counts the molecules, shows the factor-label working and splits the sample's mass across its elements.

The grams to moles formula

The link between mass and amount is the molar mass M, the mass of one mole of the substance in g/mol:

  • n = m / M: moles from grams
  • m = n × M: grams from moles
  • N = n × N_A: number of particles, with N_A = 6.02214076 × 10²³ mol⁻¹

For water, M = 2 × 1.008 + 15.999 = 18.015 g/mol, so 10.0 g of water is 10.0 / 18.015 = 0.555 mol, or about 3.34 × 10²³ molecules. Going the other way, 0.250 mol of glucose (C₆H₁₂O₆, 180.156 g/mol) weighs 0.250 × 180.156 = 45.0 g.

How to use the converter

  1. Type a formula such as NaCl, Ca(OH)2 or CuSO4·5H2O, or tap one of the common-substance chips. The "Interpreted as" line shows how the formula was read and its molar mass.
  2. Enter a mass (pg to kg) or an amount (pmol to kmol). The field you typed in last is the input, and the other one is calculated. Changing the input's unit reinterprets your number; changing the calculated field's unit re-expresses the answer.
  3. Read the result, the particle count and the dimensional-analysis chain, then copy the worked solution if you need it for homework or a lab notebook.

For a protein, polymer or a reagent sold with a quoted molecular weight, switch to Known molar mass and enter it in g/mol, kg/mol, Da or kDa. For example, 10.0 mg of a 66.5 kDa protein is 1.50 × 10⁻⁷ mol, or 150 nmol.

Factor-label method (dimensional analysis)

Textbooks teach mole conversions as a chain of fractions that each equal one. To convert 250 mg of sodium chloride to millimoles, you multiply by 1 g / 1000 mg, then by 1 mol NaCl / 58.44 g NaCl, then by 1000 mmol / 1 mol. Each unit that appears on the top of one fraction and the bottom of another cancels, leaving only mmol NaCl. The converter draws this chain with the real numbers and strikes out each cancelled pair, so you can see why the answer comes out as it does.

Ions, hydrates and tricky formulas

You can type charges in several ways: SO4^2-, SO4²⁻, SO4 2- or SO42-. The last form is ambiguous, because the 2 could be a subscript or the charge. The converter reads it as sulfate, SO₄²⁻, and offers a one-click switch to the other reading. Common singly charged ions such as NH4+ and NO3- are recognised directly. The charge does not change the molar mass in any meaningful way, because electrons are so light. Hydrates count their water of crystallisation, so CuSO₄·5H₂O has a molar mass of 249.677 g/mol, not the 159.60 g/mol of the anhydrous salt.

Capital letters matter
CO is carbon monoxide (28.01 g/mol) but Co is cobalt (58.933 g/mol). Check the "Interpreted as" line whenever a result looks wrong.

Significant figures in mole calculations

The answer is rounded to the smaller of two counts: the significant figures in your measured value and those of the molar mass. Unit factors such as 1000 mg = 1 g are exact definitions and never limit precision. A whole number such as 250 has ambiguous trailing zeros and counts as two significant figures. Write 250. or 2.50e2 to make it three. Switch to Full precision to see up to ten digits when you are checking a calculation instead of reporting it.

Radioactive elements
Technetium, promethium and most elements from polonium onward have no standard atomic weight. For these the mass number of the longest-lived isotope is used, so results are approximate.

Frequently Asked Questions

Is the Grams to Moles Converter free?

Yes, Grams to Moles Converter is totally free :)

Can I use the Grams to Moles Converter offline?

Yes, you can install the webapp as PWA.

Is it safe to use Grams to Moles Converter?

Yes, any data related to Grams to Moles Converter only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

How does this grams to moles converter work?

It works out the molar mass M from the formula you type, using IUPAC standard atomic weights, then applies n = m / M to turn a mass into moles, or m = n × M to go back. It also multiplies by Avogadro's constant (6.02214076 × 10²³ mol⁻¹) to count the particles, and shows each step as a factor-label chain.

Why does my answer differ from the answer key in the last digit?

Many textbooks and answer keys round atomic weights to two decimals (H = 1.01, O = 16.00), which gives water 18.02 g/mol instead of 18.015 g/mol. That small difference can shift the last reported digit. The answer key may also round at an intermediate step instead of only at the end.

For a hydrate, are moles of hydrate the same as moles of the anhydrous salt?

Yes. One formula unit of CuSO₄·5H₂O contains exactly one CuSO₄, so 0.0200 mol of the pentahydrate holds 0.0200 mol of CuSO₄. The masses differ, though: you must weigh more of the hydrate because the water of crystallisation adds to its molar mass.

Do ions need a different molar mass?

Practically no. An ion differs from the neutral species only by electrons, and each electron adds or removes about 0.00055 g/mol, far below the precision of the atomic weights. The converter reads charges such as SO4^2-, NH₄⁺ or Fe3+ and then uses the mass of the atoms alone.

What is the difference between moles, molecules and formula units?

A mole is a count: 6.02214076 × 10²³ of something. A molecule is a discrete covalent unit such as H₂O, while ionic solids such as NaCl have no molecules, so chemists count formula units instead. Either way, 1 mol of the substance contains Avogadro's number of those units.

How were the significant figures chosen?

The result keeps the smaller of two counts: the significant figures in the number you typed, and those of the molar mass. Unit conversions such as 1000 mg = 1 g are exact and do not limit precision. Type 250. with a trailing point, or 2.50e2, if the zero in 250 is significant, or switch to Full precision to see more digits.