Molality and Molarity

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 Molality/Molarity - The difference

Molality - number of moles of solute dissolved in one kilogram of solvent. Molality is different from molarity because molality is about concentration in kilograms of solvent and molarity is about concentration in dm3 (liters) of solvent. Molality is represented by a small 'm', whereas molarity is represented by an upper case 'M'. A 1 Molar solution (1M) contains 1 mole of solute dissolved in a solution totaling 1 liter

EXAMPLE Molarity: How to prepare 1dm3 of a 2M KCl solution using chemistry software CHEMIX School

Step 1 . Calculate the weight of  2mol KCl  (insert 2mKCl in the Formula edit field (image below))

molality and molarity

Step 2  To make a 2M aqueous solution of KCl, dissolve 149.102 grams of KCl in some distilled deionized water. Then add more water to the flask until it totals 1 liter. 

Calculate Molality

Molality and Solvents

Water as a solvent: Since water has a specific gravity of 1 (one dm3 = 1kg) , a molal concentration of 1m can be made by adding water to a volumetric flask up to the one liter line and then add the solute.

Other solvents are usually lighter than water and must therefore be weighed before adding the solute.

To make a one molal aqueous solution of potassium chloride, measure out one kilogram of water and add one mole of KCl (74.55g/mole) to it.

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