Molarity Calculator
Please provide any three values in the fields below to calculate the fourth value in the molarity equation:
Molarity = MassMolecular Weight × Volume
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 = nV
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).
If you dissolve 2 moles of sodium chloride (NaCl) in 1 liter of water, what is the molarity of the solution?
M = nV = 2 mol1 L = 2 M
This means the sodium chloride solution has a concentration of 2 M (2 moles per liter).
If you have a 1 M solution of hydrochloric acid (HCl) and need 0.5 moles of HCl, what volume of solution is required?
V = nM = 0.5 mol1 M = 0.5 L
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. Given the mass of the solute, we can calculate the number of moles of solute by dividing the mass of the solute by its molecular weight. Note that for this calculator, we use the term "molecular weight" because it is generally accepted to be equivalent to the term molar mass, especially when the molecular weight is in units of g/mol. However, the exact definition of molecular weight is not quite equivalent to molar mass, and molar mass is the more correct term. At the bottom of this page, we will provide some disambiguation regarding many of the terms used in the context of molarity.
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 = mMW V = mMW×V
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).
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:
M = mMW×V = 10 g58.44 g/mol × 0.5 L = 0.342 M
Thus, the molarity of the sodium chloride solution is 0.342 M.
Suppose you need to prepare 2 liter of a 1 M solution of potassium nitrate (KNO3). The molecular weight of KNO3 is 101.1 g/mol. How much potassium nitrate should you weigh out? First, rearrange the molarity equation to solve for the mass of solute m:
m = M×MW×V = 1 M × 101.1 g/mol × 2 L = 202.2 g
So, you need to weigh out 202.2 grams of potassium nitrate to make 2 liter of a 1 M solution.
Disambiguation of terminology
When dealing with molarity, there are many different terms used. Some of the terms used, 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 the same. Here we will do our best to define these terms and hopefully clear up any confusion.
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 6.02214076 × 1023 particles (Avogadro number).
Solvent—the substance in a solution that is present in the largest quantity. A solvent dissolves a solute. Water is a common solvent.
Solute—a solute is 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, which is equal to one dalton. Currently, after the redefinition of SI quantities in 2019, molecular weight varies very slightly from molar mass numerically. In the past, they were numerically equivalent, but with different units.
Molecular mass—the mass of a given molecule, typically in units of daltons (Da). Relative molecular mass is very close numerically to molar mass, but it 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. The main point here is to realize how the terms are being used and in what context. When used in the context of measuring molarity, each of these terms means more or less the same thing.