Molarity Vs Molality

 

Molarity Vs Molality

Molarity Vs Molality

Molarity

¨  Molarity is measuring number of moles of solute in a total volume of solution.

¨  Molarity is defined as number of moles of solute per liter of solution.

¨  Formula= moles of solute/per liter of solution

¨  Example

¨  If we have 0.5 moles of NaCl in 1.5 liter of water (H2O)

¨  Molarity will be

¨  Molarity = no of moles/ per liter of solution

¨  Molarity = 0.5/1.5 = 0.33 M

¨  Similarly if we don’t know the number of moles but have known mass then we can also find out the molarity by using mole ratio

¨  Suppose we have 400 gram of NaCl in 1.5 liter of water and we want to find Molarity

¨  We know that one mole of NaCl = 58.5 g

¨  Then

¨  400 g NaCl ₓ 1 mole of NaCl = 6.8 moles

¨             1                     58.5           

¨    6.8 moles of NaCl   = 4.5 M

¨                 1.5

Molality

¨  Molality is number of moles in relation to the mass of solvent.

¨  Molality can be defined as number of moles of solute per kilogram of solvent.

¨  Molality = number of moles/ kilogram of solvent

¨  Example

¨  Suppose we have 0.5 moles of NaCl per 1.5kg of water.

¨  Molality will be

¨  Molality = no of moles/ kilogram of solvent

¨  Molality = 0.5 / 1.5 = 0.33 mole/kg

¨  Similarly if we don’t know the number of moles but have known mass then we can also find out the molarity by using mole ratio

¨  Suppose we have 400 gram of NaCl in 1.5 kilogram of water and we want to find Molarity

¨  We know that one mole of NaCl = 58.5 g

¨  Then

¨  400 g NaCl ₓ 1 mole of NaCl = 6.8 moles

¨             1                     58.5           

¨    6.8 moles of NaCl   = 4.5 mole/kg

¨                 1.5

¨  In case of water and at low concentration there is negligible differences as we consider water density 1 (density=1).

¨  However when the solvent density is lower or greater than 1 and also the concentration is high these changes are very prominent.

¨  Example

¨  Molar and Molal volumes of 1 mole of solute dissolved in CCl4

¨  Density of CCL4 = 1.59

¨  In this case for 1 molar solution dissolve 1 mole of solute in CCl4 and make final volume up to 1 liter.

¨  For 1 molal solution in CCl4

¨  As we know mass of CCl4 = 1000 gram

¨  Density of CCl4 = 1.59

¨  Volume required = mass/density

¨  Volume required = 1000/1.59 = 629 ml

¨  We came to know that to prepare 1 molal solution in CCl4 we need to add 629ml of CCl4

Presented by

Muhammad Atif

PhD Scholar Pharmaceutical Sciences 

Abdul Wali Khan University Mardan, Pakistan