Free molarity calculator
Calculate the molarity of a solution — enter moles of solute and volume of solution in litres to see the concentration in mol/L, updated live, as you type.
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Molarity = moles of solute per liter of solution (M = mol/L).
Results are estimates. Consult a professional.
How the molarity calculator works
Molarity (M) is the amount of solute in moles divided by the volume of solution in litres. To use the calculator, provide the mass of the solute and its molar mass (from the periodic table or a formula weight lookup), then enter the volume of solution. The calculator converts mass to moles, then divides by volume. For dilution problems, the dilution equation M₁V₁ = M₂V₂ finds any one unknown given the other three.
Worked example: NaCl solution
A student dissolves 5.85 g of NaCl (table salt, molar mass = 58.44 g/mol) in enough water to make 500 mL of solution. What is the molarity?
Common solution concentrations
Standard reagent concentrations are used across chemistry, biology, and medicine. These benchmarks help you prepare solutions and interpret concentration values at a glance.
| Solution | Concentration | Typical use |
|---|---|---|
| Normal saline (NaCl) | 0.154 M (0.9% w/v) | IV fluids, contact lens solution |
| Phosphate-buffered saline (PBS) | 0.137 M NaCl | Biological buffer for cell work |
| Acetic acid (vinegar) | ≈ 0.83 M (5% w/v) | Food preservative, cleaning |
| Hydrochloric acid (conc.) | ≈ 12 M | Laboratory acid digestion |
| Sodium hydroxide (1 M) | 1.000 M | Standard base for titrations |
| Glucose (blood, fasting) | ≈ 5.0 mmol/L | Normal blood glucose reference |
| Hydrogen peroxide (3%) | ≈ 0.88 M | Household antiseptic |
| Ethanol (70%) | ≈ 12.1 M | Disinfectant / hand sanitiser |
Source: Sigma-Aldrich reagent specifications and WHO guidelines. Concentrations are approximate and vary by supplier and temperature.
Tips for molarity calculations
Lab errors in solution preparation usually trace back to unit confusion or molar-mass mistakes. These habits keep your calculations reliable.
- Look up, don't guess, molar mass — Sum atomic weights from the periodic table for each element, accounting for subscripts. H₂O = 2(1.008) + 15.999 = 18.015 g/mol.
- Volume of solution, not solvent — Molarity uses the final solution volume, not the volume of water added. Dissolve the solute first, then add water to the target volume mark.
- Dilution law shortcut — To dilute a 5 M stock to 0.1 M in 1 L: V₁ = M₂V₂/M₁ = (0.1)(1)/(5) = 0.02 L = 20 mL of stock, then top up to 1 L.
- Molarity vs. molality — Molarity (M) is moles per litre of solution; molality (m) is moles per kilogram of solvent. They differ when solute significantly changes solution volume or at non-ambient temperatures.
- Check with dimensional analysis — Track units through every step: g / (g/mol) = mol; mol / L = mol/L (M). If the units don't cancel correctly, the setup is wrong.
Accuracy and limitations
Molarity calculations are arithmetically exact. Practical accuracy depends on the precision of your mass measurement (balance accuracy), volume measurement (volumetric glassware vs. graduated cylinder), and the purity of the solute. The molar masses used here are based on 2021 IUPAC standard atomic weights and are accurate to four decimal places for common elements. Temperature affects solution volume and therefore molarity; concentrations reported without temperature are assumed at 25 °C.
Key terms
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