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

What mass of NaCl `("molar mass =58.5 g mol"^(-1))` dissolved in 50g of water will lower the freezing point by `5^@C?` The freezing point depression constant, `K_p` for water is `1.86" K kg mol"^(-1)`, van't Hoff factor for NaCl is 1.87.

Text Solution

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Here `M_(B)="58.5 g mol"^(-1), w_(A)=50g, w_(B)=?`
`triangleT_(f)=5^0 C, i=1.87, K_(f)="1.86 K kg mol"^(-1)`
Depression of freezing point, `triangleT_(f)=iK_(f) m`
`triangleT_(f)=I xx K_(f) xx (w_(B) xx 1000)/(M_(B) xx w_(A)) rArr 5=1.87 xx 1.86 xx (w_(B) xx 1000)/(58.5 xx 50)`
Mass of solute `w_(B)=(5 xx 58.5 xx 50)/(1.87 xx 1.86 xx 1000)=4.205g`
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