Discussion Forum



1)

 Consider seperate  solution of .500M    $C_{2}H_{2}OH(aq)$  ,0.100M   $Mg_{3}(PO_{4})_{2}(aq),$  0.250 M KBr(aq)  and 0.125  M    $Na_{3}PO_{4}(aq)$  at 25° C. .Which statement is true about these solution, 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 $C_{2}H_{2}OH(aq)$ has highest osmotic pressure

Answer:

Option A

Explanation:

 This problem includes concept of colligative properties (osmotic pressure here)  and van't Hoff factor. Calculate  the effective molarity  to each solution

i.e, effective molarity=van't Hoff factor x molarity

 .500M    $C_{2}H_{2}OH(aq)$       i=1

 Effective molarity  =0.5

0.250 M KBr(aq)                                 i=2

 Effective molarity=0.5 M

0.100 M $Mg_{3}(PO_{4})_{2}(aq)$ i=5

Effective molarity=0.5 M 

0.125  M $Na_{3}PO_{4}(aq)$              i=4

Effective molarity=0.5 M

 Hence, all colligative properties are same

Note This equation is solved by assuming that the examiner has taken  $Mg_{3}(PO_{4})_{2}$  to be completely soluble. However, the fact is that it is insoluble (sparingly soluble)