This molarity calculator computes molarity as moles of solute divided by liters of solution. Enter moles and liters to get concentration in mol/L (M).
Chemistry students use it when preparing solutions and checking lab math. Keep a periodic table or molar mass reference nearby when you start from grams instead of moles.
Formula
Molarity M = moles ÷ liters. Doubling moles at fixed volume doubles M. Doubling volume at fixed moles halves M.
Worked example
Moles 0.5, liters 2. Molarity = 0.5 ÷ 2 = 0.25 M.
| Input | Value |
|---|---|
| Moles of solute | 0.5 |
| Liters of solution | 2 |
| Molarity | 0.25 M |
How to use the fields
- Moles of solute is n from your weighed sample and molar mass.
- Liters of solution is the final solution volume.
From grams to moles
Moles = grams ÷ molar mass (g/mol). Weigh carefully, convert, then enter moles here. Rounding molar mass too early can shift concentration in analytical work.
Volume discipline
Use a volumetric flask for accurate liters when the procedure requires it. Graduated cylinders are coarser. If you measured milliliters, convert before you divide.
Common mistakes
- Using solvent volume instead of final solution volume
- Leaving volume in mL while calling it liters
- Confusing molarity with molality
- Forgetting hydration water in a hydrate formula mass
Lab checklist
- Write the target M on the bottle before you mix.
- Confirm solute identity and purity.
- Rinse solids into the flask so moles are not left on the weigh boat.
- Mix thoroughly before you sample concentration.
For dilutions, calculate the new molarity with M1V1 = M2V2 after both solutions use consistent volume units. This calculator stays focused on the basic M = n÷V step.
Preparing a target molarity
Decide the target M and final liters first. Multiply to get required moles, then convert moles to grams with molar mass. Weigh, dissolve, and dilute to the mark. Finally, confirm with this calculator using actual moles and liters if any amounts changed.
For hydrates, include water of hydration in the formula mass or dry the solid per procedure. Missing hydrate water throws moles off.
Dilution after stock prep
Once stock molarity is known, aliquots follow M1V1 = M2V2. Keep volume units identical on both sides. This calculator does not replace the dilution equation; it establishes M for each solution you measure.
Analytical expectations
Volumetric glassware grades differ. Class A flasks support tighter work than beakers. Match glassware to the precision your report needs.
Label bottles with solute, target M, date, and preparer initials. Future lab mates should not guess what 0.25 on a faded label meant.
Safety
Concentration math does not replace PPE and SDS guidance. Higher molarity acids and bases need handling rules beyond the quotient on this page.
Worked gram path
Suppose you need 0.25 M NaCl in 2.00 L. Required moles = 0.25 × 2.00 = 0.50 mol, which matches the default moles field. Grams = 0.50 × molar mass of NaCl. After weighing and diluting to 2.00 L, this calculator returns 0.25 M from 0.5 mol and 2 L.
If the flask is filled only to 1.90 L by mistake, molarity rises above target. Volume accuracy is as important as the weigh step.
Significant figures
Match reported digits to glassware and balance precision. Quoting many decimal places from this tool does not create precision the labware lacked.
Serial dilution overview
High concentration stocks are diluted stepwise to reach low assay concentrations. Compute each step molarity, then confirm the final moles and liters if you consolidate the story into one equivalent dilution.
Label each tube with step number and target M so trays do not get shuffled mid lab.
Common exam traps
Questions sometimes give millimoles and milliliters. Convert to moles and liters, or keep both in milli units consistently. Mixing milli on only one side breaks the quotient.
Another trap is using solvent liters from a recipe that later says dilute to volume. Always prefer final solution volume when the text offers it.
Limitations
Results ignore activity coefficients, density based conversions, and titration endpoints. They report ideal molarity from the two inputs only.
For related unit and percent checks while you plan materials, try the unit converter or the percentage calculator.