How this instrument works
Molarity (symbol M) is the most commonly used measure of solution concentration in chemistry, defined as moles of solute per liter of solution: M = moles of solute / liters of solution. A '1 M' solution means one mole of solute has been dissolved and the mixture brought up to a final total volume of exactly one liter — not one liter of solvent added to the solute, but one liter of finished solution.
That final-volume detail is the most common trap when preparing a real solution: dissolving 1 mole of solute into 1 liter of water does not generally give a 1 M solution, because the solute itself takes up some volume, pushing the total above 1 liter. Standard lab practice is to dissolve the solute in less than the target volume of solvent, then add solvent up to the calibration mark on a volumetric flask — guaranteeing the final total volume, not just the solvent volume, is exactly what the molarity calculation assumes.
Molarity's convenience is that it connects directly to volume measurements made with burettes, pipettes and graduated cylinders — the tools chemists use constantly on the bench — so a known molarity lets you measure out a specific number of moles just by measuring a volume. Its one drawback is that volume shifts slightly with temperature, so for calculations that must hold across a temperature range (like colligative properties), molality is used instead since it's anchored to mass rather than volume.
- Enter the amount of dissolved substance, in moles, into Moles of solute (mol).
- Enter the total final volume of the solution — after solvent has been added to bring it up to the mark — into Volume of solution (L).
- Read Molarity (mol/L) beneath the inputs; it recalculates the instant either value changes.
- If your volume is in milliliters, divide by 1000 before entering it, since this field expects liters.
Worked example — 0.5 mol in a 2 L final volume
Enter 0.5 into Moles of solute (mol) and 2 into Volume of solution (L) — half a mole of solute dissolved and brought up to a total final volume of 2 liters. Molarity reads 0.25 mol/L: 0.5 / 2 = 0.25.
That 0.25 M figure means each liter of this solution contains a quarter mole of solute — so measuring out exactly 1 liter with a graduated cylinder hands you 0.25 mol of solute without weighing anything further, which is the entire practical value of knowing a solution's molarity.
Questions
Is molarity the volume of solvent or the volume of the whole solution?
The whole solution — solute and solvent combined, measured after everything has been mixed together to its final total volume. This is why lab solutions are prepared in a volumetric flask: solute is dissolved in less than the target volume, then solvent is added up to the calibration mark, guaranteeing the total final volume matches what the molarity assumes rather than just the added solvent volume.
What's the difference between molarity and molality?
Molarity divides moles of solute by liters of the whole solution; molality divides moles of solute by kilograms of solvent alone. Because molarity's denominator is a volume, it shifts slightly as temperature changes and the liquid expands or contracts; molality's denominator is a mass, which doesn't change with temperature, so molality is preferred for calculations that must hold across a temperature range.
How do I dilute a solution to a target molarity?
Using the dilution relationship M1V1 = M2V2, where M1 and V1 are the concentration and volume of your starting (stock) solution and M2 and V2 are the target concentration and final volume after adding solvent. This instrument computes M directly from moles and volume; a dedicated dilution calculation instead solves that M1V1 = M2V2 relationship for whichever of the four quantities you're missing.
Why does molarity use liters rather than milliliters?
Because the formal SI-derived definition of molar concentration is moles per liter (equivalently, moles per cubic decimeter), producing typical lab values that are convenient decimal numbers — moles per milliliter would give inconveniently tiny figures for the same solution. If your volume is measured in mL, divide by 1000 before entering it here.
Can molarity be used for gases or solids, not just liquids?
The molarity concept, moles per liter, technically applies to any homogeneous mixture with a defined volume, but it's used almost exclusively for solutions of a solute in a liquid solvent. Gas-phase concentration is more commonly expressed as partial pressure (as in a Kp calculation) or as molar concentration in specific contexts, since gas volume depends strongly on pressure and temperature in ways that make 'moles per liter' less standard to quote.