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A 5% solution (by mass) of cane sugar in water has freezing point of 271 K. Calculate the freezing point of 5% glucose in water if freezing point of pure water is 273.15 K.

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A 5% solution (by mass) of cane sugar in water has freezing point of 271 K. Calculate the freezing point of 5% glucose in water if freezing point of pure water is 273.15 K.

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Here, ΔTf = (273.15 - 271) K

= 2.15 K

Molar mass of sugar (C12H22O11) = 12 × 12 + 22 × 1 + 11 × 16

= 342 g mol-1

5% solution (by mass) of cane sugar in water means 5 g of cane sugar is present in (100 - 5)g

= 95 g of water.

Now, number of moles of cane sugar = \(\frac{5}{342}\)mol

= 0.0146 mol

Therefore, molality of the solution, m = \(\frac{0.0146\ mol}{0.095\ kg}\)

= 0.1537 mol kg-1

Applying the relation,

ΔTf = Kf × m

⇒ Kf = \(\frac{ΔT_f}{m}\)

= \(\frac{2.15\ K}{0.1537\ mol\;kg^{-1}}\)

= 13.99 K kg mol - 1

Molar of glucose (C6H12O6) = 6 × 12 + 12 × 1 + 6 × 16

= 180 g mol-1

5% glucose in water means 5 g of glucose is present in (100 - 5) g = 95 g of water.

Number of moles of glucose = \(\frac{5}{180}\)mol

= 0.0278 mol

Therefore, molality of the solution,

= 0.2926 mol kg-1

Applying the relation,

ΔTf = Kf × m

= 13.99 K kg mol-1 × 0.2926 mol kg-1

= 4.09 K (approximately)

Hence, the freezing point of 5% glucose solution is (273.15 - 4.09) K = 269.06 K.

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