What is Molarity?
Molarity (M), also referred to as molar concentration, is a measure of the amount of a given substance per unit volume of a solution. It is typically measured in units of mol/L, which is often abbreviated as simply M. More specifically, in the context of a solute in solution, molarity is defined as the number of moles of solute per liter of solution.
Molarity plays a critical role in laboratory experiments, pharmaceutical preparations, and industrial chemical processes.
The Base Molarity Formula
where:
M is the molarity of the solution (mol/L)
n is the number of moles of solute (mol)
V is the volume of the solution (L)
Example: If you dissolve 2 moles of sodium chloride (NaCl) in 1 liter of water, what is the molarity of the solution?
This means the sodium chloride solution has a concentration of 2 M (2 moles per liter).
Example: If you have a 1 M solution of hydrochloric acid (HCl) and need 0.5 moles of HCl, what volume of solution is required?
So, 0.5 liters (or 500 mL) of the 1 M HCl solution is required to obtain 0.5 moles of HCl.
Measuring Molarity Using Molecular Weight
Often, we may not know the number of moles of solute directly and instead are given the mass of the solute in grams. Given the mass of the solute, we can calculate the number of moles by dividing the mass of the solute by its molecular weight.
Note: For this calculator, we use the term "molecular weight" because it is generally accepted to be equivalent to the term molar mass in practical lab settings, especially when the molecular weight is in units of g/mol.
The formula to calculate molarity directly using mass and molecular weight is:
where:
M is the molarity (mol/L)
m is the mass of the solute (g)
MW is the molecular weight of the solute (g/mol)
V is the volume of the solution (L)
Example: Suppose you have 10 grams of sodium chloride (NaCl) and dissolve it in water to form a solution of 500 mL (0.5 L). The molecular weight of NaCl is 58.44 g/mol. To find the molarity of the solution:
Disambiguation of Chemistry Terminology
When dealing with molarity, there are many different terms used. Some of the terms, while accepted for general use, are not exactly the same scientifically. Here is a breakdown of common chemistry terms:
- Molarity: The number of moles of a solute per liter of solution.
- Molar concentration: Equivalent to molarity.
- Moles: Unit of measurement in the International System of Units. It measures the amount of a substance. One mole has exactly 6.02214076 × 1023 particles (Avogadro's number).
- Solvent: The substance in a solution that is present in the largest quantity. A solvent dissolves a solute. Water is the most common solvent.
- Solute: Any substance mixed into a solvent. A solute can be comprised of gases, liquids, or solids.
- Solution: A liquid or solid mixture in which one or more solutes are dissolved by a solvent.
- Molar mass: The mass of 1 mole of a substance, typically expressed in g/mol.
- Molecular weight: Commonly used interchangeably with molar mass, but is most accurately defined as relative molecular mass: the unitless ratio of the mass of a molecule to the atomic mass constant (one dalton).
Although molar mass, molecular weight, and molecular mass are technically different physics concepts, in practical wet-lab contexts where the units are g/mol, they can be used interchangeably to measure molarity.