Question

how to calculate molarity if only known mass weight?

Answer 1

To get molarity we still need to divide moles of solute by volume of solution. But this time we're not given the molesof solute. We have to calculate it from the mass of NaCl. We multiply 2.5 g NaCl by the conversion factor of 1 mole NaCl over the formula weight ofNaCl, 58.5 g.

Answer 2

You can calculate the molarity of a solution if you know both the mass of the chemical dissolved and the volume of solution made.

Enter into the calculator the mass of chemical which was dissolved in asolvent to make the solution. This mass must be in units of grams. If your mass is in some other unit of measure (ounces or pounds, for example), you must first convert it to gram


Divide the mass of the chemical you just entered by the molecularweight of that same chemical. The molecular weight you use must be in units of grams per mole. The result of this calculation will be the number of moles of the compound in the solution.

Divide the value of molesyou just calculated by the total volume of the solution. This volume must be in units of liters.The result of this calculation is the molarity of the solution, M, in units of moles chemical per liter solution.




A common mistake students make is to divide by the volume of solvent alone. You must divide by the volume of the final mixture obtained by dissolving the chemical in the solvent, which is normally greater than the volume of the solvent alone. Strictlyspeaking, since it hasunits of grams per mole, the value described as molecular weight in this process could becalled "gram molecular weight."