What is the percent by mass of iodine needed to reduce the freezing point of benzene to `3.5^(@)"C"`? The freezing point and cryoscopic constant of pure benzene are `5.5^(@)"C"` and `5.12K/m` respectively.

Explanation

`DeltaT_f=5.2 -3.2 =2^@C`
`DeltaT_f=K_f.m=5.12m rArr m=0.39 =(g_B//M_B)/(g_A//1000)rArr (g_B)/(g_A)=(0.39xx254)/(1000)=0.099~~0.1`
`rArr % "by mass " = (g_B)/(g_A+g_B)=(0.1)/(1+0.1)xx100%=9.09%`