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NCERT Class XII Chemistry
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Question : 52
Total: 53
Calculate the mass of ascorbic acid (vitamin C,C6H8O6) to be dissolved in 75 g acetic acid to lower its melting point by 1.5°C, Kf = 3.9 K kg mol‌–1.
Solution:  
Lowering in melting point (ΔTf )=1.50
Mass of solvent ( CH3COOH),w1=75 g,
Mass of solute, w2=?
Molar mass of solvent ( CH3COOH),M1=60 g mol−1
Molar mass of solute, C6H8O6=72+8+96=176 g mol−1
(pplying the formula, ΔTf=Kf×
w2×1000
M2×w1

or w2=
M2×w1×ΔTf
1000×Kf

=
176×75×1.5
1000×3.9
=5.08 g
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