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NCERT Class XII Chemistry
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Question : 51
Total: 53
Boiling point of water at 750 mm Hg is 99.63°C. How much sucrose is to be added to 500 g of water such that it boils at 100°C ? Molal elevation constant for water is 0.52 K kg mol‌–1.
Solution:  
Elevation in boiling point (ΔTb )=100−99.63=0.370C
Mass of solvent (water), w1=500g, Mass of solute w2=?
Molar mass of solvent, M1=18 g mol−1
Molar mass of solute, C12 H22 O11=342 g mol−1
Applying the formula, ΔTb=Kb×
w2×1000
M2×w1

or, w2=
M2×w1×ΔTb
1000×Kb

=
342×500×0.37
1000×0.52
=122 g
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