Molarity Calculator · Practical guide
How to Calculate Molarity from Grams
Two quick divisions turn a mass in grams into a molarity in mol/L.
At a glance
The Formula
To calculate molarity from grams, first convert the mass to moles by dividing by the molecular weight, then divide by the solution volume in liters: molarity = (grams ÷ molecular weight) ÷ liters. For example, 10 g of NaCl (58.44 g/mol) dissolved to make 500 mL of solution gives 0.1711 mol in 0.5 L, which is 0.3422 M.
Interactive tool
Molarity Calculator
Enter your own values, then compare the result with the examples below.
Results
Step by Step
- Find the moles: 10 g ÷ 58.44 g/mol = 0.1711 mol.
- Convert the volume to liters: 500 mL ÷ 1,000 = 0.5 L.
- Divide moles by liters: 0.1711 ÷ 0.5 = 0.3422 mol/L.
Worked Examples
The same three steps cover any solute and volume:
| Solute | Mass | Molecular Weight | Volume | Molarity |
|---|---|---|---|---|
| NaCl | 5.844 g | 58.44 g/mol | 1 L | 0.1000 M |
| NaCl | 10 g | 58.44 g/mol | 500 mL | 0.3422 M |
| Glucose | 18.02 g | 180.16 g/mol | 250 mL | 0.4000 M |
| NaOH | 25 g | 40.00 g/mol | 2 L | 0.3125 M |
Common Mistakes to Avoid
- Forgetting to convert mL to L — molarity is always per liter of solution.
- Using the volume of solvent you started with instead of the final solution volume; dissolving a solute changes the total volume slightly.
- Using the wrong molecular weight — look it up or calculate it from the chemical formula before you start.
Frequently Asked Questions
Do I need the molecular weight to find molarity from grams?
Yes. Grams alone cannot be converted to moles without the molecular weight (g/mol), which you can get from the periodic table or a molecular weight calculator.
How do I convert milliliters to liters?
Divide the volume in milliliters by 1,000. So 250 mL is 0.25 L and 750 mL is 0.75 L.
What is the difference between molarity and molality?
Molarity is moles of solute per liter of solution, while molality is moles of solute per kilogram of solvent. Molality does not change with temperature; molarity does, slightly.
Why is my answer slightly different from a textbook value?
Differences usually come from rounding the molecular weight (for example, 58.44 versus 58.5 for NaCl) or rounding intermediate mole values. Carry four decimal places until the final step.