Molarity Calculator
Please provide any three values in the fields below to calculate the fourth value in the molarity equation:
Molarity = MassMolecular Weight × Volume
Related: Molecular Weight Calculator
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 formula for molarity is M = n / V, where M is the molarity of the solution (mol/L), n is the number of moles of solute (mol), and V is the volume of the solution (L).
Example:
If you dissolve 2 moles of sodium chloride (NaCl) in 1 liter of water, the molarity of the solution is M = 2 mol / 1 L = 2 M.
Example:
If you have a 1 M solution of hydrochloric acid (HCl) and need 0.5 moles of HCl, the volume required is V = n / M = 0.5 mol / 1 M = 0.5 L (500 mL).
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. Given the mass, we can calculate the number of moles by dividing the mass by its molecular weight. For this calculator, we use the term "molecular weight" because it is generally accepted to be equivalent to molar mass, especially when the molecular weight is in units of g/mol (the exact definitions differ slightly, as explained below).
Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). The formula to calculate molarity using molecular weight is M = m / (MW × V), where m is the mass of the solute (g), MW is the molecular weight (g/mol), and V is the volume (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. The molarity is M = 10 / (58.44 × 0.5) = 0.342 M.
Example:
To prepare 2 liters of a 1 M solution of potassium nitrate (KNO3, molecular weight 101.1 g/mol), rearrange the equation to solve for the mass: m = M × MW × V = 1 M × 101.1 g/mol × 2 L = 202.2 g of potassium nitrate.
Disambiguation of terminology
When dealing with molarity, many terms are used. Some, while accepted for general use, are not exactly the same. In particular, "molecular weight" is often used interchangeably with "molar mass," but the two are not actually identical.
- Molarity — the number of moles of a solute per liter of solution.
- Molar concentration — equivalent to molarity.
- Moles — the SI unit measuring the amount of a substance. One mole has 6.02214076 × 1023 particles (the Avogadro number).
- Solvent — the substance in a solution present in the largest quantity; it dissolves a solute. Water is a common solvent.
- Solute — any substance mixed into a solvent; it can be a gas, liquid, or solid.
- 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 most accurately defined as relative molecular mass: the unitless ratio of the mass of a molecule to the atomic mass constant, equal to one dalton. Since the 2019 redefinition of SI quantities, molecular weight varies very slightly from molar mass numerically; in the past, they were numerically equivalent, with different units.
- Molecular mass — the mass of a given molecule, typically in daltons (Da). It is very close numerically to molar mass, but takes into account the masses of each nuclide in a molecule, while molar mass uses the standard atomic weights of each element.
Although molar mass, molecular weight, and molecular mass are not exactly the same, in less formal contexts where the units and quantities do not have to be absolutely correct, they can be used largely interchangeably.