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Molar Concentration

Molar Concentration

What is molar concentration? How to calculate it? Read on, to find all the answers. . .
ScienceStruck Staff
In chemistry, as in all sciences, it's essential that one can 'quantify' things through experimentation. When you take up your first physical chemistry lab class, you will realize that it's a science which gives a lot of importance to precision. To be able to successfully carry out chemical reactions, it's essential that the ingredient or reactant chemicals are mixed in just the right proportions. Of the many parameters that characterize a chemical solution, one of the most important ones is molar concentration.

A mole is a unit for specifying the quantity of a chemical substance just like a 'Kilo' is a unit for specifying mass of any substance. A mole of any chemical substance is the molecular weight of that substance, directly expressed in the unit of grams.

One mole of any solute substance, exactly contains 6.023 x 1023 molecules. So taking a mole of a substance is exactly quantifying the number of molecules that you are using in your chemical reaction.

The molar concentration or molarity of a specific chemical in a solution can be defined to be the number of moles dissolved in unit volume of the solution. In other words, it defines how many moles of any solute, will exist in unit volume of its solution. The commonly used unit for it is 'Mol/Liter' or Mol/L.

Here is the molarity formula, which you must remember or be able to deduce, to calculate molar concentration accurately.

Molar Concentration = (Number of Moles of Solute) / (Volume of Solution) = n / V = (Mass of Solute in grams) / (Volume of Solution in Liters x Molar Mass of Solute in Grams)

The above formula can also be used for calculating molar concentration of ions and molecules.

To execute the calculation, you need to know two parameters. One is the number of moles of solute and the second is the volume of the solution. To be able to calculate the moles of a substance or solute, first calculate its molecular weight, by extracting the atomic weight values of its constituent atoms.

The molar mass of the solute is the molecular weight of the solute expressed in grams. Check out this article which explains how to calculate molar mass.

Once you know the molar mass, calculating the number of moles is simple. Deduce the number of moles by dividing the total mass of the solute by the molar mass. To get the molar concentration, simply divide the number of moles (which may be a fractional number), by the volume of the solution in liters.

Solved Example
Suppose that about 117 gm NaCl (common salt) is dissolved in two liter (V) of distilled water. What will be the molar concentration of salt in the water solution?

Firstly, you need to know what is the molecular weight of NaCl, which will be the sum of the atomic weights of sodium (Na) and chlorine (Cl). Looking at the periodic table of elements, one finds that the atomic weight of Na and Cl is 23 and 35.5 respectively. So the molecular weight of NaCl will be (23 + 35.5) = 58.5. So the molar mass of NaCl is 58.5 gm.

So the number (n) of moles of NaCl in 117 gm of common salt will be (117 / 58.5) = 2 moles. Therefore the molar concentration of 117 gm of NaCl is equal to:

Molar Concentration = n / V = (2 moles of NaCl) / (2 Liters of Water) = 1 Mol/L

To sum up, molar concentration is the parameter, which tells you how diluted a chemical is, in its solution. Practice calculation for sucrose, common salt or any other such chemical, to master the entire calculation procedure, as you will need to use it regularly during your lab course.