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Let w2 and M2 = Masses and molar masses of solute w1 = Mass of solvent
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 .
Sol . Elevation of boiling point ΔTb = ( 100 + 273 ) - ( 99.63 + 273 ) = 0.37 K
Mass of water , wl = 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 :
= ( 0.37 x 342 x 500 ) / ( 0.52 x 1000 ) = 121.67 g ( approximately ) Hence , 121.67 g of sucrose is to be added .