Calculate the mass of a non-volatile (Molar-mass 40 g 1 mol−
) which should be
dissolved in 114 g octane to reduce its vapour pressure to 80%.
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Answer:
Let p be the vapour pressure of pure octane. The vapour pressure of solution will be
100
80
p=0.8 p.
Molar mass of solute (M) and octane (m) are 40 g/mol and 114 g/mol respectively. Mass of octane, w is 114 g.
p
p−p
′
=
Mw
Wm
p
p−0.8p
=
40×114
W×114
W=8g
Hence, 8 g of solute are required.
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