What mass of NaCl must be dissolved in 65.0 g of water to lower the freezing point of water by `7.50^@C` ? The freezing point depression constant (Kg) for water is `1.86^@`C/m. Assume van't Hoff factor for NaCl is 1.87. [Molar mass of NaCl = 58.5 g]

Explanation

van.t Hoff factor = Observed depression in freezing point / Calculated depression in freezing point
1.87 = 7.5/Calculated depression
Calculated depression = 7.5/1.87 = 4
Now apply the relation
`M_2 = (K_f xx w_2 xx 1000)/(Delta T_f xx w_1)`
`58.5 = (1.86 xx w_2 xx 1000)/(4 xx 65)`
`w_2 = (58.5 xx 4 xx 65)/(1.86 xx 1000) = 8.177 g`