5 g of `Na_(2)SO_(4)` was dissolved in x g of `H_(2)O` . The change in freezing point was found to be `3.82^(@)C` . If `Na_(2)SO_(4)` is `81.5%` ionised , the value of x
(`k_(f)` for water =`1.86^(@)C` kg `"mol"^(-1)`) is apporximately :
(molar mass of S=32 g `"mol"^(-1)` and that of Na=23 g `"mol"^(-1)`)

Options

A. 15 g
B. 45 g
C. 25 g
D. 65 g

Explanation

`{:(" "Na_(2)SO_(4)to2Na^(+)2Na^(+)+SO_(4)^(2-)), ("intial " " "1"mol" " "0" "0),("After ionisation"" "1-alpha" "2alpha" "alpha):}`
Total number of moles `=1+2alpha`
`i=1+2 alphaimplies1+2xx0.815=2.63`
`:'DeltaT_(f)=(1000xxK_(f)xxw_(2)xxi)/(M_(2)xxw_(1))`
`3.82=(1.86xx2.63xx5xx1000)/(142xx x)" " :. " "x=(1.86xx2.63xx5000)/(142xx3.82)=45g`