Molarity Calculator · Practical guide
How to Convert Grams to Moles
One division — grams divided by molecular weight — converts any mass to moles.
At a glance
The Short Answer
To convert grams to moles, divide the mass by the substance's molecular weight: moles = grams ÷ molecular weight (g/mol). For example, 100 g of water (18.015 g/mol) is 100 ÷ 18.015 = 5.551 mol. The same one-step formula works for every pure substance.
Interactive tool
Molarity Calculator
Enter your own values, then compare the result with the examples below.
Results
Why the Conversion Matters
Chemical reactions happen particle to particle, not gram to gram. A mole is a fixed count of particles (6.022 × 10^23), so converting to moles lets you compare substances on equal footing — one mole of NaCl reacts with one mole of AgNO3 even though their masses differ completely.
100 Grams of Common Substances
| Substance | Molecular Weight (g/mol) | Moles in 100 g |
|---|---|---|
| Water (H2O) | 18.015 | 5.551 mol |
| Carbon dioxide (CO2) | 44.009 | 2.272 mol |
| Sodium chloride (NaCl) | 58.44 | 1.711 mol |
| Glucose (C6H12O6) | 180.16 | 0.555 mol |
Worked Example
Suppose a recipe calls for 0.25 mol of NaCl and you have a scale: 0.25 mol × 58.44 g/mol = 14.61 g. Going the other way, if you weighed out 14.61 g and want the moles: 14.61 ÷ 58.44 = 0.25 mol. Multiplication and division by the molecular weight move you back and forth between the two units.
From Moles to Molarity
Once you have moles, one more division gives concentration: molarity = moles ÷ liters of solution. Those 1.711 mol of NaCl dissolved to make 2 L of solution produce a 1.711 ÷ 2 = 0.856 M solution. The calculator above performs both conversions at once.
Frequently Asked Questions
Where do I find the molecular weight?
Add up the atomic weights of every atom in the chemical formula using periodic table values, such as H = 1.008 and O = 15.999. For NaCl it is 22.990 + 35.45 = 58.44 g/mol.
How do I convert moles back to grams?
Multiply by the molecular weight: grams = moles × g/mol. Half a mole of water is 0.5 × 18.015 = 9.01 g.
Does the grams-to-moles conversion depend on temperature?
No. Mass and moles are independent of temperature. Only volume-based measures like molarity shift slightly as liquids expand or contract.
What is a mole in simple terms?
A mole is a counting unit: 6.022 × 10^23 particles, chosen so that one mole of a substance weighs its atomic or molecular weight in grams.