Molality
∴ Number of moles of CCl4 = 70 / 154 mol = 0.4545 mol
Mole fraction of C6H6 =
Molality
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Molality( m) can be defined as the ratio of number of moles of solute to the mass of solvent in kg.
It can also be defined as the number of moles of solute present in unit kilogram of solvent.
Mathematically, Molality( m) = Moles of Solute / Mass of Solvent in Kg
For instance, molality of a solution containing 74.5 g( 1 mol) of KCl dissolved in 1 kg of water is 1.00 m or 1.00 mol kg – 1.
Problem: Calculate the mass of urea( NH2CONH2) required in making 2.5 kg of 0.25 molal aqueous solution.
Solution:
Molar mass of urea( NH2CONH2) = 2( 1 × 14 + 2 × 1) + 1 × 12 + 1 × 16 = 60 g mol- 1
0.25 Molar aqueous solution of urea means 1000 g of water contains 0.25 mol =( 0.25 × 60) g of urea = 15 g of urea
( 1000 + 15) g of solution contains 15 g of urea.
Therefore, 2.5 kg( 2500 g) of solution contains =( 15 X 2500) /( 1000 + 15) g = 36.95 g = 37 g of urea( approximately).
Mass of urea required = 37 g