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Question
Calculate the mass of ascorbic acid (Vitamin C, C6H8O6) to be dissolved in 75 g of acetic acid to lower its melting point by 1.5°C. Kf = 3.9 K kg mol−1.
Solution
Lowering in melting point (ΔTf) = 1.5°C
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), M2 = 6 × 12 + 8 × 1 + 6 × 16
= 72 + 8 + 96
= 176 g mol−1
Kf = 3.9 K kg mol−1
Applying the formula,
`Delta"T"_"f" = ("K"_"f"xx"w"_2xx1000)/("M"_2xx"w"_1)`
`=>"w"_2=(Delta"T"_"f"xx"M"_2xx"w"_1)/("K"_"f"xx1000)`
`= (1.5xx176xx75) /(3.9xx1000)`
= 5.08 g (approx)
Hence, 5.08 g of ascorbic acid is needed to be dissolved.
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