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 approximately:

(molar mass of S = 32g $mol_{−1}$ and that of Na = 23g $mol_{−1}$)