Both molarity and molality are measures of concentration, or how much solute is dissolved in solvent. Let's first get acquainted with a couple of terms.
Solute - what gets dissolved
Solvent - what the solute dissolves in
Solution - solute and solute together
Now, we can write out formulas for our two terms.
Molarity(M) = moles of solute/ Liters of solution
Molality(m) = moles of solute/ kg of solvent
As evident from the formulas, molarity and molality equations both have the same top term. However their denominators are different. For molarity calculation, we need to find liters of solution and divide by it.
For molality, we need to find kilograms of solvent.
The units for molarity are mol/L or M
The units for molality are mol/kg
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