At \(T(K)\), \(x\) g of a non-volatile solid (molar mass \(78\ \mathrm{g\ mol^{-1}}\)) when added to \(0.5\) kg water, lowered its freezing point by \(1.0^\circ C\). What is \(x\) (in g)?
\[
K_f \text{ of water at } T(K)=1.86\ \mathrm{K\ kg\ mol^{-1}}
\]