Molarity by Mass and Volume

Molarity from mass and volume

Molarity (M, mol/L) measures how much solute sits in a liter of solution. When you start from mass, the route is M = (m / MW) / V, with m the solute mass (g), MW the molar weight (g/mol) and V the solution volume (L). Take 5.85 g of NaCl (MW = 58.5 g/mol) in 1 L: that's 5.85/58.5 = 0.1 mol, so 0.1 M. The SI unit is mol/m³, though mol/L (M) is what labs actually use. Because it hangs on volume, molarity shifts a little with temperature as the liquid expands.

Applications

Analytical chemistry leans on it for titration, calibration curves and standard solutions. Clinical labs report electrolytes in mEq/L and glucose in mmol/L. In agrochemistry you convert fertilizer and pesticide doses from g/L to mol/L. It also turns up in pharmaceutical formulations, brines and isotonic solutions like saline 0.9% and glucose 5%.

FAQ

Solution volume or solvent volume? The total volume of the solution once the solute has dissolved, read off a volumetric flask. It is never the volume of pure solvent you poured in.

Why does molarity depend on temperature? Heat expands the solvent, so the same count of moles now fills a larger volume and M drops. Molality (mol/kg) sidesteps the whole issue.

How to convert g/L to mol/L? Divide the mass concentration by the molar weight, so g/L ÷ g/mol = mol/L.