A non -volatile solute.'A' tetramerises in water to the extent of 80% .2.5 g of 'A' in 100 g of water .lower the freezing point by 0.3 `""^(@)C`.The molar mass of Å in mol `4^(-1)` is (`K_(T)`For water =1.86 K kg `mol^(-1)`)

Options

A. 62
B. 221
C. 155
D. 354

Answer

A

Explanation

(a): As non-volatile solute ‘A’ tetramerises,
`:. alpha =(l=i) (n)/(n-1) alpha = 80%` (given)
0.8 = (l-i) `4/3 rArr i=0.4`
`Delta T_(f) = iK_(f)m`
`0.3=0.4xx1.86xx (2.5)/("Molar mass") xx (1000)/(100)`
Molar mass `= (0.4xx1.86xx2.5xx10)/(0.3)` = 62 g/mol
Note: Read mole `L^(-1)` as g `mol^(-1)` in question.