How to Calculate Molarity from Molecular Weight

In the field of chemistry, molarity is the most commonly used term to define the concentration of any chemicals or solutions. It is the unit of concentration which is widely used for making stock solutions, dilutions etc which are normally used for finding the purity and quality of any research sample or for different means. No matter where these molar solutions are used, it is very essential that there molarity should be calculated in a correct manner to make the result precise and calculations as robust as possible. Mole is one other term which is used in chemistry and refers to the ratio of mass in grams of solute and its molecular weight. So by the knowledge of volume of the final solution as well as the mass of solute which is dissolved in that solution, molarity of a solution can easily be calculated.

Things that are needed for calculating the molarity of a solution by the help of molecular weight include:

  • Mass of the solute dissolved in the solution.
  • Molecular weight of the solute.
  • Calculator

Steps for calculating the molarity of from molecular weight:

  1. Mass of the solute that is dissolved in the solvent to make the final solution is measured and entered in the calculator. It should be noted that the mass of solute dissolved should be in the units of grams. If mass is available in some other units of weight for e.g. milli grams, pounds, ounces, micro grams etc. then necessary calculations should be done for the conversion of such units into grams and then it should be entered in the calculator.
  2. Mass of solute which is entered in the calculator is then divided by the molecular weight of the similar compound which is dissolved in the solvent i.e. the same of whom you e=entered amount in grams per mole. It should be noted that the molecular weight of solute dissolved should be in the units of grams. Now press equal key and it will give the result which will be the number of moles of solute present in the solution.
  3. Now this calculated value of number of moles of solute are dived with the total volume of solution in liters. It should be noted that the volume of solution should be in the units of liters.
  4. Again press the equal key of calculator. The result that appears on the display is the required molarity of the solution which is calculated by means of its molecular weight.

Instructions & caution:

  • Most of the students commit the same mistake i.e. they divide the number of moles with the volume of solvent alone rather than to divide it with the volume of whole solution i.e. solute plus solvent.
  • Necessary calculations for the conversion of units should be prior to the real calculations of molarity of a solution.

Here’s an Easy to use Molarity Calculator for you to calculate molarity online!