The concentrations of solutions can be expressed in number of ways; viz : mass fraction of solute (or mass percent), Molar concentration (Molarity) and Molal concentration (molality). These terms are known as concentration terms and also they are related with each other i.e. knowing one concentration term for the solution, we can find other concentration terms also. The definition of different concentration terms are given below :
Molarity : It is number of moles of solute present in one litre of the solution.
Molality : It is the number of moles of solute present in one kg of the solvent
Mole Fraction = 
If molality of the solution is given as 'a' then mole fraction of the solute can be calculated by
Mole Fraction = 
; = 
where a = molality and M solvent = Molar mass of solvent
We can change : Mole fraction ↔ Molality ↔ Molarity
(i) 60 g of solution containing 40% by mass of NaCl are mixed with 100 g of a solution containing 15% by mass NaCl. Determine the mass percent of sodium chloride in the final solution.
Text Solution
Verified by ExpertsCHECK THE SOLUTION
(i) % of (w/w) = 
= 
× 100 = 24.4%.
(ii) Mass of solute = 60 × 0.4 + 100 × 0.15 = 24 + 15 = 39 g
Mass of solvent = 160 – 39 = 121 g
Molality = 
= 5.509 = 5.5 m.
(iii) Mass of solute = 39 g
Volume of solution = 
= 100 ml
∴ Molarity = 
= 6.67 M
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