Molarity CalculatorConcentration, grams needed and volume in mol/L
Solve any molarity problem three ways, correct for reagent purity, and get a bench-ready recipe for making the solution.
Molar masses follow IUPAC standard atomic weights; the definition follows the IUPAC Gold Book.
By Manzoor Ahmad, web development expert · Updated 0.0750 mol NaCl for 500 mL at 0.150 M
grams = M × V × MW grams = 0.150 × 0.500 L × 58.44 grams = 4.383 g
What is molarity?
Molarity (M) is the moles of solute per liter of final solution: M = n ÷ V. To find the grams you need, multiply molarity × volume in liters × molar mass. For example, 500 mL of 0.150 M sodium chloride needs 0.150 × 0.500 × 58.44 = 4.38 g of NaCl, dissolved and then made up to exactly 500 mL.
Molarity formula: three ways to rearrange it
Every molarity question uses the same relationship between mass, moles, volume and molar mass. Pick the arrangement that matches what you are solving for.
Here m is mass in grams, MW is molar mass in g/mol and V is the final volume in liters. If your reagent is less than 100% pure, divide the mass you weigh by the purity fraction (for 98% pure, divide by 0.98).
How to use the molarity calculator
- Choose what you need: grams to weigh, the molarity of something you already made, or the volume to dissolve into.
- Pick the compound or enter a custom molar mass. Use the hydrate value if you are weighing a hydrate.
- Enter your numbers with their units. mL, µL, mg and mM are converted for you.
- Set purity from the bottle label if it is below 100%.
- Read the result, conversions and recipe. Copy them into your lab notebook with one click.
Worked example: making 500 mL of 0.150 M saline
This is the calculator’s default example. A pharmacy or teaching lab needs 500 mL of 0.150 M sodium chloride (molar mass 58.44 g/mol).
- Convert volume to liters: 500 mL ÷ 1,000 = 0.500 L.
- Find moles needed: 0.150 mol/L × 0.500 L = 0.0750 mol.
- Convert moles to grams: 0.0750 mol × 58.44 g/mol = 4.383 g.
- Express the strength other ways: 0.150 × 58.44 = 8.77 g/L, which is 0.877% w/v.
For comparison, 0.9% w/v normal saline is 9.0 g/L ÷ 58.44 = 0.154 M. In practice, keeping both forms in mind (molar and percent) catches labeling mistakes quickly, because the two numbers should always agree.
Grams needed for common laboratory solutions
Mass to weigh for two common preparations, using the same engine as the calculator (100% purity).
| Compound | Molar mass (g/mol) | 1 L of 1 M | 100 mL of 0.1 M |
|---|---|---|---|
| Sodium chloride (NaCl) | 58.44 | 58.44 g | 0.5844 g |
| Potassium chloride (KCl) | 74.551 | 74.55 g | 0.7455 g |
| Sodium hydroxide (NaOH) | 39.997 | 40.00 g | 0.4000 g |
| Glucose (C6H12O6) | 180.156 | 180.2 g | 1.802 g |
| Tris base | 121.14 | 121.1 g | 1.211 g |
| Calcium chloride (CaCl2) | 110.98 | 111.0 g | 1.110 g |
| Sodium bicarbonate (NaHCO3) | 84.007 | 84.01 g | 0.8401 g |
| Copper(II) sulfate·5H2O | 249.69 | 249.7 g | 2.497 g |
Molarity vs molality vs normality vs % w/v
Concentration units answer slightly different questions. Choosing the wrong one is a frequent source of exam and lab errors.
| Unit | Definition | Changes with temperature? | Typical use |
|---|---|---|---|
| Molarity (M) | mol solute per L of solution | Slightly, yes | Lab solutions, titrations, kinetics |
| Molality (m) | mol solute per kg of solvent | No | Boiling point elevation, freezing point depression |
| Normality (N) | equivalents per L of solution | Slightly, yes | Acid–base and redox titrations |
| % w/v | grams per 100 mL of solution | Slightly, yes | Pharmacy and clinical labeling |
For freezing point and boiling point work, use the molality calculator, since molality is defined by solvent mass rather than solution volume.
Common molarity mistakes to avoid
Plugging 250 instead of 0.250 into M = n/V makes the answer 1,000 times too small.
Molarity is per liter of final solution. Dissolve, then fill to the mark.
CuSO4·5H2O weighs 56% more per mole than anhydrous CuSO4.
A 95% pure reagent needs 5.3% more mass to reach the same molarity.
Molarity calculator FAQs
Molarity (M) is the number of moles of solute dissolved per liter of solution. A 1 M solution contains 1 mole of solute in every 1 liter of the final solution, not 1 liter of solvent.
Molarity = moles of solute ÷ liters of solution, or M = n/V. Because moles = mass ÷ molar mass, you can also write M = mass ÷ (molar mass × volume in liters).
Multiply the target molarity by the volume in liters and by the molar mass: grams = M × V × MW. For 500 mL of 0.150 M NaCl, that is 0.150 × 0.500 × 58.44 = 4.38 g.
Divide milliliters by 1,000. 250 mL is 0.250 L. The calculator converts mL and µL automatically, which removes the most common molarity mistake.
Molarity is moles per liter of solution and changes slightly with temperature because volume changes. Molality is moles per kilogram of solvent and does not change with temperature.
If a reagent is 98% pure, only 98% of what you weigh is the compound you want. Divide the theoretical mass by the purity fraction: 4.383 g ÷ 0.98 = 4.47 g.
mM is millimolar, one thousandth of a mole per liter (0.001 M). µM is micromolar, one millionth of a mole per liter (0.000001 M). 1 M = 1,000 mM = 1,000,000 µM.
Multiply molarity by the molar mass to get g/L, which equals mg/mL. A 0.150 M NaCl solution is 0.150 × 58.44 = 8.77 g/L, or 8.77 mg/mL.
Percent w/v is grams per 100 mL. Take g/L and divide by 10. 0.154 M NaCl is about 9.0 g/L, which is 0.9% w/v normal saline.
Yes, if you are weighing the hydrated form. Copper(II) sulfate pentahydrate has a molar mass of 249.69 g/mol, while anhydrous CuSO4 is 159.61 g/mol. Using the wrong one changes the concentration by over 50%.
The solute takes up volume. Dissolving first in about 80% of the final volume, then filling to the calibration line, gives the exact final volume that molarity is defined on.
The math is exact and molar masses use IUPAC standard atomic weights. Real accuracy depends on your balance, glassware and reagent purity, so verify critical preparations against your lab's protocol.
Sources
This molarity calculator is an educational and planning tool. Verify clinical, pharmaceutical or safety-critical preparations against your facility’s validated procedures and an independent second check.