Molarity Definition

Molarity = moles of solute ÷ liters of solution (mol/L): the standard concentration unit used across chemistry.

M = \frac{n_{\text{solute}}}{V_{\text{solution}}}

Molarity = moles of solute / liters of solution

What it means

Molarity counts how many moles of solute sit in each liter of the finished solution. It is the concentration chemists reach for first in stoichiometry, titrations, and dilutions, because solution volumes are easy to measure and the mole is the unit reactions think in. Remember the denominator is the total solution volume after everything is dissolved and diluted to the mark — not the solvent you started with.

Variables

Symbol	Name	Unit	Meaning
$M$	Molarity	mol/L (M)	Concentration of the solution: moles of solute per liter of solution.
$n_solute$	Moles of solute	mol	The amount of solute dissolved, in moles.
$V_solution$	Volume of solution	L	The total volume of the finished solution, after diluting to the mark.

Assumptions

V is the volume of the final solution, not the volume of solvent added.
Volume changes with temperature, so molarity is temperature-dependent.
Uppercase M is the molarity unit; lowercase m is molality — a different quantity.
The solute dissolves completely without reacting.

Worked example

Problem

A chemist dissolves 5.85 g of NaCl and dilutes it to a final volume of 250.0 mL. What is the molarity of the solution?

Given:

Mass of NaCl = 5.85 g
Final solution volume = 250.0 mL

Molar mass of NaCl = 22.990 + 35.45 = 58.44 g/mol.
Convert mass to moles: n = 5.85 g / 58.44 g/mol = 0.100 mol.
Convert volume to liters: 250.0 mL = 0.2500 L.
M = n / V = 0.100 mol / 0.2500 L = 0.400 M.

0.400 M NaCl

Each liter of this solution contains 0.400 mol of NaCl — a typical working concentration for a lab bench solution.