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What mass of NaCI ("molar mass" =58.5g m...

What mass of `NaCI ("molar mass" =58.5g mol^(-1))` be dissolved in `65g` of water to tower the freezing point by `7.5^(@)C`? The freezing point depression constant, `K_(f)`, for water is `1.86 K kg mol^(-1)`. Assume van't Hoff factor for `NaCI` is `1.87`.

Text Solution

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`Delta T_(f) , i= (K_(f) xx 1000 xx w_(B))/(w_(A) xx M_(B))`
` I = 1.87`
`k_(f) = 1.86 KT kg mol^(-1)`
`w_(A) = 65.0 g , M_(B) =58.5 ,Delta T_(f) = 7.50^(@)C`
`7.50 = 187 xx (1.86 xx 1000 xx w_(B))/(65.0 xx 58.5)`
`w_(B) = (7.50 xx 65.0 xx 58.5)/(1.87 xx 1.86 xx 1000)`
` = 8.2 g`
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