Molarity Definition in Chemistry

What Molarity Means (With Examples)

scientist looking at solution in beaker
Molarity is the moles of solute per liter of solution. Glow Images, Inc / Getty Images

In chemistry, molarity is a concentration unit, defined to be the number of moles of solute divided by the number of liters of solution.

Units of Molarity

Molarity is expressed in units of moles per liter (mol/L). It's such a common unit, it has its own symbol, which is a capital letter M. A solution that has the concentration 5 mol/L would be called a 5 M solution or said to have a concentration value of 5 molar.

Molarity Examples

  • There are 6 moles of HCl in one liter of 6 molar HCl or 6 M HCl.
  • There are 0.05 moles of NaCl in 500 ml of a 0.1 M NaCl solution. (The calculation of moles of ions depends on their solubility.)
  • There are 0.1 moles of Na+ ions in one liter of a 0.1 M NaCl solution (aqueous).

Example Problem

Express the concentration of a solution of 1.2 grams of KCl in 250 ml of water.

In order to solve the problem, you need to convert the values into the units of molarity, which are moles and liters. Start by converting grams of potassium chloride (KCl) into moles. To do this, look up the atomic masses of the elements on the periodic table. The atomic mass is the mass in grams of 1 mole of atoms.

mass of K = 39,10 g/mol
mass of Cl = 35.45 g/mol

So, the mass of one mole of KCl is:

mass of KCl = mass of K + mass of Cl
mass of KCl = 39.10 g + 35.45 g
mass of KCl = 74.55 g/mol

You have 1.2 grams of KCl, so you need to find how many moles that is:

moles KCl = (1.2 g KCl)(1 mol/74.55 g)
moles KCl = 0.0161 mol

Now, you know how many moles of solute are present. Next, you need to convert the volume of solvent (water) from ml to L. Remember, there are 1000 milliliters in 1 liter:

liters of water = (250 ml)(1 L/1000 ml)
liters of water = 0.25 L

Finally, you're ready to determine molarity.

Simply express the concentration of KCl in water in terms of moles solute (KCl) per liters of solute (water):

molarity of solution = mol KC/L water
molarity = 0.0161 mol KCl/0.25 L water
molarity of the solution = 0.0644 M (calculator)

Since you were given mass and volume using 2 significant figures, you should report molarity in 2 sig figs also:

molarity of KCl solution = 0.064 M

Advantages and Disadvantages of Using Molarity

There are two big advantages of using molarity to express concentration. The first advantage is that it's easy and convenient to use because the solute may be measured in grams, converted into moles, and mixed with a volume.

The second advantage is that the sum of the molar concentrations is the total molar concentration. This permits calculations of density and ionic strength.

The big disadvantage of molarity is that it changes according to temperature. This is because the volume of a liquid is affected by temperature. If measurements are all performed at a single temperature (e.g., room temperature), this is not a problem. However, it's good practice to report the temperature when citing a molarity value. When making a solution, keep in mind, molarity will change slightly if you use a hot or cold solvent, yet store the final solution at a different temperature.