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02/01/2022 4:24 pm
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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.
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02/01/2022 4:33 pm
Here, elevation of boiling point ΔTb = (100 + 273) - (99.63 + 273)
= 0.37 K
Mass of water, w1 = 500 g
Molar mass of sucrose (C12H22O11), M2 = 11 × 12 + 22 × 1 + 11 × 16 = 342 g mol-1
Molal elevation constant, Kb = 0.52 K kg mol-1
We know that,
ΔTb = \(\frac{K_b \times 1000 \times w_2}{M_2 \times w_1}\)
⇒ w2 = \(\frac{\Delta T_b \times M_2 \times w_1}{K_b \times 1000}\)
= \(\frac{0.37 \times 342 \times 500}{0.52 \times 1000}\)
= 121.67 g (approximately)
Hence, 121.67 g of sucrose is to be added.