Answer: 0.028 grams
Explanation:
Depression in freezing point :
Formula used for lowering in freezing point is,
[tex]\Delta T_f=k_f\times m[/tex]
or,
[tex]\Delta T_f=k_f\times \frac{\text{ Mass of solute in g}\times 1000}{\text {Molar mass of solute}\times \text{ Mass of solvent in g}}[/tex]
where,
[tex]\Delta T_f[/tex] = change in freezing point
[tex]k_f[/tex] = freezing point constant (for benzene} =[tex]5.12^0Ckg/mol[/tex]
m = molality
Putting in the values we get:
[tex]0.400^0C=5.12\times \frac{\text{ Mass of solute in g}\times 1000}{354.5\times 209.0}[/tex]
[tex]{\text{ Mass of solute in g}}=0.028g[/tex]
0.028 grams of DDT (solute) must be dissolved in 209.0 grams of benzene to reduce the freezing point by 0.400°C.
