How to Calculate Molarity

Chemistry is a branch of science with a lot of terminologies and notations. Let us try to know more about one such term - molarity. Chemists define molarity as the measure of the concentration of a solute in a solution. To calculate molarity, you can remember this formula:

Molarity =	No. of Moles of Solute
	Volume of Solution in Liters

Thus, the unit of molarity is moles/liter, written as mol/L. So, while calculating molarity, we need to first calculate the number of moles of a solute. But, what is the number of moles? Mole number or number of moles is defined as the ratio of mass of the solute to the molar mass of the solute. Thus, by substituting this in the above equation, the formula for molarity becomes:

Molarity =	Mass of the Solute
	Molar Mass of the Solute X Volume of Solution in Liters

Now, another question erupts - do you know what is meant by molar mass of a solute? It's important to first discuss that. The molar mass of any element/compound is defined as the atomic/molecular weight expressed in grams. Take the example of Carbon, whose atomic number is 12. So, the molar mass of carbon is 12 gms. Molar mass is important because if we take the molar mass of any substance, then it has 6.0221415×10²³ (Avogadro's constant) number of atoms or molecules of that substance (which can be a chemical element or a compound). So, this means that 12 gms of carbon will have 6.0221415×10²³ atoms of carbon.

Now, what about molar mass of a compound? An example, straight away, will clear up things. Consider Sodium Chloride (NaCl), whose molecular weight is 58.5 (23 for Na + 35.5 for Cl). So, in case of compounds, the molecular weight expressed in gms is the molar mass, which in this case is 58.5 gms. In other words, 58.5 gms of NaCl has 6.0221415×10²³ molecules.

Calculation of Molarity

The standard formula for calculating molarity is the first one, so we''ll be using that expression. To proceed, we first need to find the molar mass of the solute. For that you need to refer to a periodic table with atomic masses and note down the atomic mass of the required elements.

Now that we have the molar mass, we can go ahead and find the molarity. Let us follow the academic way and take an exercise to better illustrate the calculation of molarity.

Example: Calculate the molarity of a solution prepared by dissolving 25 gms of common salt in 500 mL of water.

Solution:
First we calculate the number of moles of solute present:

no. of moles of NaCl =	25
	molar mass of NaCl

=	25
	58.5

= 0.427 moles.

No. of liters of water = 500 mL = 0.5 L
Now, from the first formula.

Molarity =	No. of Moles of NaCl
	Volume of Water in Liters

Molarity, M =	0.427
	0.5

= 0.854 mol/L

Similarly, you can try to find the molarity in the following examples, and cross-check them with the answers given:

• Calculate the molarity of a solution containing 1.724 moles of H₂SO₄ as the solute in 2.50 L of solvent? (Answer: 0.69 mol/L)
• Find out the molarity of a solution made by dissolving 25.0 g of HCl in water to make 150.0 mL of solution? (Answer: 4.57 mol/L)

I hope now you have understood how to calculate molarity. It's one of the most important characteristics of a solution.