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Question
Calculate the mass of a non-volatile solute (molar mass 40 g mol−1) which should be dissolved in 114 g octane to reduce its vapour pressure to 80%.
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Solution
`(p^circ - p)/p^circ = (w M)/(W M')`
or `1 - p/p^circ = (w M)/(W M')`
By substituting the given values, we get
`1 - 80/100 = (w xx 114)/(114 xx 40)` ...(Mol. mass of octane = 114)
or `20/100 = w/40`
or `w = (20 xx 40)/100`
w = 8 g
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