Molarity Calculator
Moles per litre of solution — not of solvent.
Work out Molarity. Moles per litre of solution — not of solvent. Says which constants are exact and which are conventions.
58.44 for NaCl, 180.16 for glucose.
Molarity (mol/L)
1
1,000 mM · 1 mol
Molarity is moles per litre of solution — not of solvent, which is the distinction that trips people. Dissolving something changes the volume, so a 1 M solution is made by dissolving the solute and then topping up to the mark, never by adding it to a full litre of water.
How the Molarity Calculator works
Molarity is moles of solute per litre of solution, and that word matters: dissolving something changes the volume, so a 1 M solution is made by dissolving the solute and topping up to the mark, never by adding it to a full litre of water.
Also known as: molar concentration calculator · moles per litre calculator · how to calculate molarity · molarity formula
Of solution, not of solvent
Molarity is moles per litre of the finished solution. Dissolving something changes the volume, so adding one mole to one litre of water does not give a 1 M solution — it gives slightly more than a litre of something slightly weaker.
The correct method is to dissolve the solute in less than the target volume, then top up to the mark. That is what a volumetric flask is for, and why its neck is narrow.
Molarity against molality
Molality is moles per kilogram of solvent, and the difference matters when temperature changes. Volume expands with heat; mass does not.
So a solution's molarity drifts as it warms while its molality stays fixed. For work at a single temperature molarity is more convenient; across a range, molality is the honest measure.
Frequently asked questions
How do I calculate molarity?
Divide the moles of solute by the litres of solution. 58.44 g of sodium chloride is one mole, so in one litre it makes a 1 M solution.
What is the difference between molarity and molality?
Molarity is per litre of solution and molality is per kilogram of solvent. Molality does not change with temperature, since mass does not expand, which is why it is preferred for work across a temperature range.
How do I find moles from grams?
Divide the mass by the molar mass. 29.22 g of a substance with a molar mass of 58.44 g/mol is 0.5 moles.
What does mM mean?
Millimolar — thousandths of a mole per litre. It is the working unit for most of biology, where concentrations are far below 1 M.
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