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NCERT Class XII Chemistry
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Question : 18
Total: 53
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%.
Solution:  
Number of moles of solute =
w
40
 ‌mol
Let P0=100 then Ps=80,ΔP=20
No. of moles of solvent (octane) =
114 g
114 g mol−1
=1 mol
[molar mass of C8H18=114 g mol−1]
Now,
ΔP
P0
=x2‌∴‌
20
100
=
w
40
w
40
+1
or, 0.2 (
w
40
+1 )=
w
40
‌‌ or,
0.8w
40
=0.2 or w=10 g
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