Overview
Molarity is one of the most common ways chemists express concentration: moles of solute (the substance being dissolved) per liter of total solution. It is written as M, and shows up constantly in lab work, from preparing reagents to interpreting a solution's label.
This guide covers the core formula, the most common unit-conversion mistake, and why molarity is not the same thing as the similarly-named molality.
Moles per liter
Molarity (M) is defined as moles of solute divided by liters of solution: M = n / V, where n is moles of solute and V is the total volume of solution in liters — not just the volume of solvent added before the solute. This distinction matters because dissolving a solute changes the total solution volume, and molarity is always based on the final, total volume.
Convert milliliters
Laboratory glassware is very often marked in milliliters, but the molarity formula requires volume in liters. Divide any milliliter measurement by 1,000 before using M = n/V — for example, 250 mL becomes 0.25 L. Skipping this conversion is the single most common error when calculating molarity by hand, and it throws the answer off by a factor of 1,000.
Not the same as molality
Molarity and molality are easy to confuse because they sound alike and both measure concentration, but they are defined differently: molarity uses liters of total solution (M = mol/L), while molality uses kilograms of solvent only (m = mol/kg). Molality is preferred in situations where temperature changes are involved, since volume changes with temperature but mass does not — molarity, by contrast, is more common in everyday lab work and is what most calculators, including this one, compute.
A worked example
Suppose you dissolve 0.5 moles of a solute into enough water to make 2 liters of total solution. Molarity is 0.5 / 2 = 0.25 M. If instead you were given 20 grams of a solute with a molar mass of 40 g/mol, you would first convert grams to moles (20 / 40 = 0.5 moles) before applying the same M = n/V formula.
Frequently asked questions
How do I convert grams to moles for a molarity calculation?
Divide the mass of the solute (in grams) by its molar mass (in grams per mole, found from the periodic table or a compound's molecular formula). The result is the number of moles to use as n in the molarity formula.
Does temperature affect molarity?
Yes, slightly — because molarity depends on solution volume, and most liquids expand a small amount as they warm, a solution's molarity can shift very slightly with temperature. This is one reason molality (based on mass, which does not change with temperature) is preferred for precise work across a range of temperatures.
What is the difference between concentration and molarity?
Concentration is the general term for how much solute is present in a solution, and it can be expressed several ways — molarity, molality, percent by mass, and others. Molarity is simply the most common specific unit of concentration, defined as moles of solute per liter of solution.
Try the calculator
Use the free Molarity Calculator for live numbers, then come back to this guide anytime you need a quick refresher.