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15 g of an unknow molecular substance is...

15 g of an unknow molecular substance is dissolved in 450 g of water. The reulting solution freezen at `-0.34^(@)C`. What is the molar mass of the substance (`K_(f)` for water=1.86 K kg `mol^(-1)`)

Text Solution

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The correct Answer is:
182.35 g `mol^(-1)`
`W_(B)=15.0g, W_(A)=540g, 0.45kg DeltaT_(f)=0-(-0.34)^(@)C`
`=0.34^(@)C=0.34 K, K_(f)=1.86" K kg mol"^(-1)`
`M_(B)=(K_(f)xxW_(B))/(DeltaT_(f)xxW_(A))((1.86" K kg mol"^(-1))xx(15.0g))/((0.34K)xx(0.45 kg))=182.35" g mol"^(-1)`
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