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Molarity Calculator

What molarity is

Molarity (M) is the concentration unit chemists reach for first: moles of solute per liter of solution. The formula is M = n/V, with n in moles and V in liters. Note that the denominator is total solution volume, not solvent volume — once solute dissolves, the volume includes everything in the flask. Rearranged forms cover the other two cases: n = M × V to find moles from a known concentration and volume, and V = n/M to find what volume contains a given amount.

The non-obvious twist is unit handling. Volumes in the lab are usually in mL while M is defined in mol/L, so a conversion step (mL → L by dividing by 1000) sneaks into nearly every calculation. If you start from grams instead of moles, there’s a second conversion: divide by molar mass to get n. This calculator handles both internally and shows the conversions in the steps panel so you can check them.

What the calculator does

  1. Enter any two of: molarity, moles, volume.
  2. Leave the unknown blank. The calculator picks the right rearrangement and solves.
  3. If you only know grams of solute (not moles), enter the mass plus the molar mass — it converts to moles first, then to molarity.
  4. All unit conversions (mL ↔ L, g ↔ mol) are shown step-by-step.

Worked examples

Find molarity from grams. 5.85 g NaCl (M = 58.44) dissolved to 500 mL of solution.

  • Moles: 5.85 / 58.44 = 0.1001 mol
  • Volume: 500 mL = 0.500 L
  • Molarity: 0.1001 / 0.500 = 0.200 M

Find moles from M and V. 250 mL of 0.5 M HCl.

  • n = M × V = 0.5 × 0.250 = 0.125 mol

Find volume from M and n. Volume of 2.0 M NaOH containing 0.30 mol.

  • V = n/M = 0.30 / 2.0 = 0.15 L = 150 mL

Grams to molarity. 4.00 g NaOH (M = 40.00) in 200 mL of solution.

  • Moles: 4.00 / 40.00 = 0.100 mol
  • Volume: 0.200 L
  • Molarity: 0.100 / 0.200 = 0.500 M

Where molarity gets used

Solution preparation (weigh out grams to hit a target M), titration calculations (the endpoint condition is moles_acid = moles_base, both expressed as M × V), reaction stoichiometry in solution (multiply M × V to get moles, then apply the mole ratio from the balanced equation), and the starting point for dilution problems via C1V1 = C2V2. Drug dosing and buffer preparation in biology and medicine all run on molarity for the same reason: M × V is the cleanest way to count molecules in a flask.

Frequently Asked Questions

What is molarity?
Molarity (M) is moles of solute per liter of solution — emphasis on solution, not solvent. A 1 M NaCl solution contains 1 mole of NaCl in every liter of the final mixed solution, after the salt has dissolved and the volume has settled. Molarity has units of mol/L, sometimes written M, and it is the most common concentration unit in solution chemistry because it makes solution stoichiometry direct.
How do you calculate molarity?
M = n/V, with n in moles and V in liters. If you have grams of solute instead of moles, divide by the molar mass first to get n, then divide by V. The volume is total solution volume, not solvent volume — adding solute changes the total. For dilute aqueous solutions the difference is small; for concentrated solutions or non-aqueous solvents it can matter.
What is the difference between molarity and molality?
Molarity (M) is moles per liter of solution. Molality (m) is moles per kilogram of solvent. The difference is the denominator: solution volume vs. solvent mass. Molality is temperature-independent (mass doesn't change with temperature) while molarity drifts as the solution expands or contracts. For colligative-property calculations (freezing-point depression, boiling-point elevation) molality is the right unit.
Can I use this calculator to find volume or moles?
Yes. M = n/V has three variables; supply any two and the calculator solves for the third. n = M × V (moles from concentration and volume), V = n/M (volume from moles and concentration), M = n/V (concentration from moles and volume). The same widget runs all three rearrangements.
What units should I use for volume?
Liters or milliliters — the calculator accepts both and converts to liters internally because molarity is defined as mol/L. The 'Show Steps' panel includes the unit conversion explicitly. Mixing units (entering volume in mL while expecting an answer in mol/L) is the most common source of errors in molarity problems, so the conversion step is shown for verification.