19.5 g of fluoro acetic acid (molar mass $=78 \mathrm{~g} \mathrm{~mol}^{-1}$ ) is dissolved in 500 g of water at 298 K . The depression in the freezing point of water was $1^{\circ} \mathrm{C}$. What is $\mathrm{K}_{\mathrm{a}}$ of fluoro acetic acid ? (For water, $\mathrm{K}_{\mathrm{f}}= 1.86 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$ ).Assume molarity and molality to have same values.