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