Molarity is a measure of solution concentration that expresses the number of moles of dissolved solute per liter of solution. It is calculated by dividing the amount of solute in moles by the total solution volume in liters and is commonly expressed in moles per liter (mol/L), abbreviated M.
In chemistry and chemical reactions, molarity provides a standard way to measure and compare solutions. If you’ve ever mixed a sports drink, brewed coffee, or diluted a cleaning solution, you’ve adjusted concentration in a solution — even if you didn’t call it that.
The Basic Definition of Molarity
Molarity describes how much of a dissolved substance is present in a specific volume of solution. Understanding the difference between solute, solvent, and solution makes the definition easier to apply.
- Solute: The substance being dissolved, such as sodium chloride or sugar.
- Solvent: The substance that dissolves the solute, usually water in introductory chemistry examples.
- Solution: The homogeneous mixture formed when the solute dissolves in the solvent.
For example, a solution containing one mole of sodium chloride in a final volume of one liter has a molarity of 1 M. The volume refers to the entire prepared solution, not simply the amount of water added.
Molarity also connects solution measurements to chemical equations. For practical examples involving reactant quantities, see Stoichiometric Calculations.
What Is a Mole?
A mole is the SI unit used to measure the amount of a substance. One mole contains exactly 6.02214076 × 10²³ specified elementary entities, which may be atoms, molecules, ions, or other particles.
This number is known as Avogadro’s number. It allows chemists to relate microscopic particles to quantities large enough to measure in a laboratory.
The concept is explained further in What Is a Mole in Chemistry?, including how moles connect particle counts and mass.
Moles are also essential to Stoichiometry, which uses balanced chemical equations to calculate how much reactant is needed or how much product can form.
The Molarity Formula and Its Units
The formula for calculating molarity is M = n ÷ V, where M represents molarity in mol/L, n represents the amount of solute in moles, and V represents the total volume of solution in liters.
For example, dissolving 0.50 mole of solute to prepare 1.00 liter of solution gives a molarity of 0.50 M. If the same amount of solute is present in only 0.50 liter of solution, the molarity increases to 1.00 M.
Volume must be expressed in liters when using this formula. To convert milliliters to liters, divide by 1,000. Thus, 250 mL equals 0.250 L.
Worked Example: Calculating Molarity From Mass
Suppose a student dissolves 5.85 grams of sodium chloride (NaCl) in water and prepares a final solution volume of 500 mL. What is the molarity?
- Find the molar mass: Sodium chloride has a molar mass of approximately 58.44 g/mol.
- Convert grams to moles: 5.85 g ÷ 58.44 g/mol ≈ 0.100 mol.
- Convert volume to liters: 500 mL ÷ 1,000 = 0.500 L.
- Calculate molarity: 0.100 mol ÷ 0.500 L = 0.200 M.
The prepared sodium chloride solution therefore has a molarity of approximately 0.200 M. This means each liter of the solution would contain 0.200 mole of NaCl formula units at the same concentration.
How to Prepare a Solution With a Known Molarity
Laboratories commonly prepare solutions by weighing a calculated mass of solute, dissolving it in solvent, and adjusting the final volume using a volumetric flask.
For a 0.100 M sodium chloride solution with a final volume of 1.00 L, the required amount is 0.100 mol. Multiplying by NaCl’s molar mass of 58.44 g/mol gives 5.844 g of sodium chloride.
The solute is dissolved in less than one liter of water, then additional water is added until the solution reaches the one-liter calibration mark. Adding the salt directly to exactly one liter of water would not guarantee a final solution volume of one liter.
How Dilution Changes Molarity
Dilution decreases molarity by increasing solution volume without changing the amount of dissolved solute. When no solute is lost or chemically changed, the relationship is expressed as M₁V₁ = M₂V₂.
Here, M₁ and V₁ are the initial molarity and volume, while M₂ and V₂ represent the final molarity and volume.
For example, suppose 100 mL of a 2.0 M solution is diluted to a final volume of 500 mL. The new molarity is (2.0 × 100) ÷ 500 = 0.40 M. The solute amount remains constant, but it is distributed through five times the original volume.
Molarity Compared With Other Concentration Units
Molarity is one of several ways to express concentration. Different units are useful depending on whether a measurement is based on solution volume, solvent mass, or the proportion of a substance in a mixture.
| Concentration Unit | Definition | Common Use |
|---|---|---|
| Molarity (M) | Moles of solute per liter of solution | Reaction calculations and laboratory solutions |
| Molality (m) | Moles of solute per kilogram of solvent | Temperature-dependent solution studies |
| Mass percent | Solute mass divided by total solution mass, multiplied by 100 | Commercial mixtures and chemical formulations |
| Mass concentration | Mass of solute per volume of solution, such as g/L | Water analysis and laboratory measurements |
Unlike molarity, molality does not depend on solution volume. This makes molality particularly useful when temperature changes could cause a liquid to expand or contract.
What Factors Affect Molarity?
Molarity depends directly on the amount of solute and inversely on solution volume. Adding more solute increases molarity if the final volume does not increase proportionally, while adding solvent generally decreases it.
Temperature can also affect molarity because liquids expand or contract as temperature changes. If the amount of dissolved solute remains constant but the solution volume increases, its molarity decreases slightly.
Evaporation can increase molarity by removing solvent, provided the solute remains dissolved. Chemical reactions, precipitation, or solute loss can also change the concentration and must be considered separately.
Where Molarity Is Used
Molarity is widely used in chemical laboratories to prepare standard solutions, perform titrations, and calculate reactant quantities. Knowing solution concentration allows researchers to repeat experiments under controlled conditions.
It is also useful in pharmaceutical manufacturing, environmental testing, and biological research. For example, laboratories measure dissolved substances in water samples and prepare solutions with specified concentrations for analytical procedures.
Although many consumer products use percentages or other concentration units on their labels, molarity provides a convenient way to connect dissolved chemical quantities directly to reaction equations.
Frequently Asked Questions
Is molarity the same as concentration?
Molarity is one specific way to express concentration. Concentration can also be measured using mass percent, molality, grams per liter, and other units, depending on the substance and application.
Can molarity be greater than 1 M?
Yes. A solution containing two moles of solute per liter has a molarity of 2 M. The maximum achievable molarity depends on the solute’s solubility, the solvent, temperature, and other conditions.
Does adding water always decrease molarity?
Adding water generally decreases the molarity of an aqueous solution when the amount of dissolved solute stays constant and the total volume increases. If a chemical reaction occurs or the solute amount changes, the simple dilution relationship may no longer apply.
Molarity connects the amount of dissolved substance to the total volume of a solution, making it useful for preparing mixtures and calculating chemical reactions. To solve molarity problems accurately, convert volumes to liters, distinguish solution volume from solvent volume, and check whether the amount of solute changes during dilution.
Sources: IUPAC Compendium of Chemical Terminology (Gold Book); Theodore L. Brown and colleagues, Chemistry: The Central Science; NIST Guide for the Use of the International System of Units (SI).
