Molarity Calculator
Calculate molarity (mol/L) from moles and volume, or from mass and molar mass. Essential for chemistry lab work. Also outputs mg/mL concentration.
How to use this tool
- Enter moles of solute (n), volume of solution, mass of solute (optional) and molar mass of solute (optional) in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your molarity and the full breakdown beneath it.
Molarity (M) = moles of solute / litres of solution. If you know the mass and molar mass instead, calculate moles first: n = mass / M_r, then M = n / V.
Formula
M = n ÷ V
If mass and molar mass are given: n = mass (g) ÷ molar mass (g/mol), then M = n ÷ V
Concentration in mg/mL = M × molar mass
Where M = molarity (mol/L), n = moles of solute, V = volume of solution (L).
How it works
This calculator computes the molar concentration of a solution — the number of moles of solute dissolved per litre of solution. It accepts either moles directly, or mass plus molar mass from which it derives the moles. It also converts molarity to mg/mL when the molar mass is provided. The formula assumes a homogeneous solution and that the total volume is the final volume of the solution, not just the solvent.
Worked example
- Given: moles of solute n = 0.5 mol, volume of solution V = 1 L.
- Apply: M = n ÷ V = 0.5 ÷ 1.
- Calculate: M = 0.5 mol/L.
Molarity = 0.5 mol/L; Moles of solute = 0.5 mol
Common mistakes to avoid
- Entering total solution mass instead of volume -- molarity is mol per litre of solution, not per kilogram of solvent.
- Forgetting to convert volume from mL to L -- 250 mL = 0.250 L; entering 250 inflates the molarity by 1000x.
- Using an approximate molar mass instead of the precise atomic-weight sum, causing systematic error especially for compounds with many atoms.
Key terms
- Molarity (M)
- The concentration of a solution expressed as moles of solute per litre of solution, with units mol/L (also written as M).
- Mole (mol)
- The SI unit for amount of substance. One mole contains ~6.022 × 1023 formula units of a substance.
- Molar mass
- The mass of one mole of a substance in grams per mole (g/mol), numerically equal to the substance's molecular (or formula) weight.
- Solute
- The substance dissolved in a solution — for example, NaCl dissolved in water.
- Solution volume
- The total volume of the final solution (solute + solvent combined), not just the volume of the solvent alone.
Frequently asked questions
- What is the difference between molarity and molality?
- Molarity (M) is moles per litre of solution; molality (m) is moles per kilogram of solvent. Molality doesn't change with temperature; molarity can because liquids expand.
- How do I make a 1 M NaCl solution?
- Dissolve 58.44 g of NaCl (molar mass ≈ 58.44 g/mol) in enough water to make exactly 1 litre of solution.