Question

Consider separate solutions of `0.500 M C_(2)H_(5)OH(aq)`,`0.100 M Mg_(3)(PO_(4))(aq)`,`0.250 M KBr(aq)`, and `0.125 M Na_(3)PO_(4)(aq)` at `25^(@)C`. Which statement is true about these solutions, assuming all salts to be strong electrolytes?
A. They all have the same osmotic pressure
B. `0.100 M Mg_(3)(PO_(4))_(2)(aq)` has the highest osmotic pressure
C. `0.125 M Na_(3)PO_(4)(aq)` has the highest osmotic pressure
D. `0.500 M C_(2)H_(5)OH(aq)` has the highest osmotic pressure

Answer 1

Correct Answer - A
`{:(0.5 M C_(2)H_(5)OH(aq),0.1M Mg_(3)(PO_(4))_(2)(aq)),(i=1,i=5),("effective molarity "=0.5,"effective molarity "=0.5 m),(0.25 M KBr (aq),0.125 M Na_(3)PO_(4)(aq)),(i=2,i=4),("effective molarity "=0.5 M,"effective molarity "=0.5 M):}`
Hence all colligative properties are same.
Note : This equation is solved by assuming that the examinar has taken `Mg_(3)(PO_(4))_(2)` to be completely soluble. However the fact is that it is insoluble (sparingly soluble).