Calculating Molarity

Science cannot give us any accurate results or concrete evidence to validate theories, unless we quantify our results and have the ability to measure the experimental variables. The slightest of variation from expected numerical results may require us to reformulate our theories. In chemistry, when synthesizing chemicals, what matters is the proportion in which reactants are mixed to get a particular resultant. Of the many ways in which one could quantify or measure the degree of dilution of a solute, in a solvent, one is the measurement of molarity.

What is Molarity of a Solution?

Any definition is based on certain primary concepts, which one needs to know. To know what molarity means, one needs to know what is a 'mole' of a substance. One mole of any substance is its molecular weight or atomic weight exactly expressed in grams. Like a dozen apples are always 12 in number , one mole of any substance, exactly contains 6.023 x 10²³ atoms or molecules. This quantity is known as Avogadro's number. Now that you know what is a mole, defining molarity is simple. Molarity is defined for a solution, with respect to a solute dissolved in a solution. Molarity is the number of moles of a solute, contained in a unit liter of that solution. It is a numerical value of how many moles of the solute are present in one liter of a particular solution. Refer to the article on how to calculate molar mass, to know how to calculate moles of a solute.

Formula for Calculating Molarity

From the definition above, the molarity formula used for its calculation can be easily guessed. The formula is as follows:

Molarity of a solution= n / V = N / (N_A x V)

In the above formula, n is the number of moles of solute, V is the total volume of the solvent, N number of molecules of solute and N_A is the Avogadro's number.

Ways to Calculate Molarity

There are various ways in which you can calculate molarity of a solution from different known parameters. Let us see how to calculate molarity in various ways.

Using Standard Formula
If you know the quantity (mass) of solute dissolved in a solution and the volume of a solution, you can use the above standard formula to calculate molarity. First you must determine the number of moles a given amount of solute. For that, you need to determine the molecular weight of the solute. Then divide the number of moles by the volume of solvent in liters. The result will be the molarity of the solution.

From Density
Calculating molarity of a solution is possible if you know its density and molecular weight of the solute. Using the following formula, molarity calculation from density, is possible.

Molarity of a Solution = [Density (in gm / liter) / (1 mole of solute)]

In short, divide the density of the solute by value of one mole of the solute to get the molarity.

From pH
Molarity calculation of ions and more precisely Hydrogen ions is possible, from the pH of the solution. The formula for pH is:

pH = - log [Molarity of H⁺ ions]

From this formula, you can reverse compute the molarity of H⁺ ions. Using the following formula:

Molarity of H⁺ ions = 10^(-pH)

Molarity calculation using the standard formula is the simplest method. However, you need to know other ways of calculating molarity indirectly, which have been presented above.