Molarity Calculator

Molarity

There are many ways to describe the concentration of a substance (the solute) in a solution. For example, we know that sea water contains salt, but you may be interested in how much salt it contains. There are multiple ways to express this quantity relative to the water. We can say that seawater consists of 35 grams of salt per liter of seawater. We can also say there are 35 parts of salt per thousand parts of seawater. This concentration can also be expressed as 3.5%.

Molarity is a way for chemists to describe the concentration of a substance relative to the solvent (the liquid it is dissolved in). The measurement unit of molarity, denoted as M, is expressed in mol/L.

A mole is the SI (International System of Units) standard for measuring the number of some individual units that make up a substance. Just like one dozen represents the number twelve, so does one mole represent another number. This number is Avogadro's number and is equal to 6.02214076*10²³. So, where a dozen eggs is twelve eggs, a mole of eggs would be 6.02214076*10²³ eggs.

As you can read, one mole is a large quantity and is therefore used to represent extremely small units, such as atoms or molecules. For example, one mole of water molecules, with the chemical formula H₂O, means 6.02214076*10²³ molecules of water.

This is useful, because the mass of different molecules and atoms differ. Gold has a greater mass than hydrogen and one gram of gold contains far fewer atoms than one gram of hydrogen. By using moles, we standardize the amounts. 1 mol of gold has the same amount of gold atoms as 1 mol of hydrogen has hydrogen atoms, even though their mass will differ enormously.

This calculator

To calculate the molarity of a chemical compound, you need to know its molar mass and the its concentration in some other unit of measurement. Usually chemists weigh their compound and dissolve it in some volume of a solvent. Thus, this calculator allows to convert the concentration of a compound in mg/ml to molarity and vice versa.

The equation to calculate molarity is as follows:

Example

David weighs 35 mg of NaCl (table salt) and dissolved it in 700 ml water. The molar mass of NaCl = 58.44 g/mol¹. What is the molarity of the solution?

1. Express concentration in mg per ml: 35 mg / 700 ml = 0.05mg/ml.

2. Convert this concentration to g/L. One gram is 1000 mg, so we convert the mg to g: 0.05/1000g/ml = 0.00005g/ml.

3. One liter is 1000 ml. So we convert the ml to liters: 0.00005g/1000ml = 0.05g/L.

4. Finally we divide the concentration in g/L with the molar mass in g/mol to get the molarity: 0.05g/L / 58.44g/mol = 0.0008555783 mol\L 7 or 8.556e-4 M.

¹To calculate the molar mass of NaCl you can use our molar mass calculator, or try to calculate it yourself after reading the explanation on that page.