1.

What mass of NaCl must be dissolved in 65.0 g of water to lower the freezing point of waterby 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]

Answer»

Solution : 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`


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