How many moles of sucrose should be dissolved it 500 g of water so as to get a solution which has 3 difference of $104°\mathrm{C}$ between boiling point and freezing point? $\left(K_f=1.86\mathrm{K} \mathrm{kg} {\mathrm{mol}}^{-1},K_b=0.52\mathrm{K} \mathrm{kg}\right.mo{l}^{-1})$