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Calculate the mass of a non-volatile sol...

Calculate the mass of a non-volatile solute ( molar mass 40g `mol^-1`) which should be dissolved in 114g octane to reduce its vapour pressure to 80%.

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Verified by Experts

`p^(@) = 10 (say), p_(s) = 80, , n_(B) = ?`
`n_(A) = (W_(A))/(M_(A)) =(114 g)/( 114 g mol^(-1)) = 1 ` mol
[`M_(A)` : Molar mass of octane , `C_(8) H_(18)`
`= 12 xx 8 + 18`
`= 114 g mol^(-1)` ]
`(p^(@) - p_(S))/(p^(@)) = (n_(B))/(n_(B) + n_(A))`
` (100 - 80)/( 100) = (n_(B))/( n_(B) + 1 mol )`
`(n_(B) + 1 mol )/( n_(B)) = 5`
`(1 + (1 mol)/( n_(B)) = 5 rArr n_(B)) = (1)/(4) mol`
But `n_(B) = (W_(B))/( M_(B))`
`:. W_(B) = n_(B) xx M_(B)`
`= (1)/(4) mol xx 40 g mol ^(-1)`
= 10 g
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