Arrange the following solutions in increasing order of their osmotic pressures .
(i)34.2 g/litre surcrose
(ii)60 g/litre of urea
(iii)90 g/litre of glucose
(iv)58.5 g/litre of sodium chloride

Explanation

Molar mass of sucrose `(C_(12)H_(22)O_(11))="342 g mol"^(-1)`
Molar mass of urea `(NH_(2)CONH_(2))="60 g mol"^(-1)," Molar mass of glucose "(C_(6)H_(12)O_(6))="180 g mol"^(-1)`
Molar mass of `NaCl="58.8 g mol"^(-1),`
Molar conc. Of sucrose `=(34.2)/(342)=0.1M," Molar conc of urea "=(60)/(60)=1M,`
Molar conc of glucose `=(90)/(180)=0.5M," Molar conc. of NaCl"=(58.5)/(58.5)=1M,`
However, as NaCl is an electrolyte and one formula unit of NaCl dissociates to give two ions (`Na^(+) and Cl^(-)`) therefore, molar concentration of particles in the solution = 2M. Thus, the order or increasing concentration is
`underset("(0.1 M)")("Sucrose")" "lt" "underset("(0.5 M)")("Glucose")" "lt" "underset("(1 M)")("Urea")" "lt" "underset("(2 M)")("NaCl")`
As osmotic pressure (or any colligative property) is directly proportional to the number of particles in the solution, hence increasing order of osmotic pressure will be : `"Sucrose" lt"Glucose"lt"Urea"lt "NaCl"`
Note. If the above solutions are to be arranged in order of their decreasing freezing points, the depression in freezing points `(DeltaT_(f))` will be in the order : `"Sucrose "lt "Glucose"lt"Urea"lt"NaCl"`
Thus, sucrose will have minimum depression i.e., its actual freezing point will be maximum. Hence, the order of decreasing freezing points will be : `"Sucrose"gt "Glucose"gt "Urea"gt"NaCl"`