Explain why solutions with the same concentration of different solutes have different osmotic pressures?

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jerichorayel | College Teacher | (Level 2) Senior Educator

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Osmotic pressure is applied in a solution to prevent the flow of water through a semi permeable membrane. It is colligative properties where in the properties of the particular solution depend on ratio of the number of moles of solute with the solvent in the solution. However, not all solutions of the same concentration would have the same value for the osmotic pressure. There is a factor called Van't Hoff factor which actually tell how much of the solute is actually present in the solution. Why do this matter? 

Not all solutes are the same in terms of their dissolution with the solvent. Some of which are electrolytic (separate into ion in the solution) and others are non-electrolytic (do not dissociates into ions). For example, if you dissolve 1 mole of glucose (C6H12O6) in 500mL of water, the Van't Hoff factor is equal to 1 since glucose does not dissociate into ions. On the other hand, 1 mole of Sodium chloride (NaCl) that is dissolved in 500mL of water will have a Van't Hoff factor of 2. It is because NaCl dissociates into Sodium and chloride ions in the solution of water. 

The formula of osmotic pressure is expressed as:

Osmotic pressure = iMRT


M = concentration in molarity

R = gas constant

T = temperature

i = Van't Hoff factor