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