Grade 12Physical Chemistry
The v.p. of water and diluted aq.solution are 24mmHg and 22.88mmHg .the molarity is
The v.p. of water and diluted aq.solution are 24mmHg and 22.88mmHg .the molarity is
1 Answer
Samyak Jain
Let P0 and PS denote vapour pressure of pure water and diluted aqueous solution respectively.
Let nB and nA denote number of moles of solute and solvent(water) in the solution.
P0 = 24 mm of Hg and PS = 22.88 mm of Hg.
XB = mole fraction of solute = nB / (nA + nB).
By Roult’s law, (P0 – PS) / P0 = nB / (nA + nB). For diluted aqueous solutions, nA + nB 
nA
i.e. (P0 – PS) / P0 = nB / nA [But actual formula is (P0 – PS) / PS = nB / nA]
Mass of water in solution = 18 nA g , density of water = 1 g/ml = 1000 g/L.
Volume of water = mass/density = (18 nA / 1000) L = (9 nA / 500) L = Volume of the solution.
Since no. of moles of solute is very less, it would not contribute significantly in mass of the solution.
Molarity of solution = No. of moles of solute / Volume of solution in L
= nB / (18 nA / 1000) = (14nA / 300) / (9nA / 500) = (14)(5) / (9)(3)
= 70 / 27 2.6 M.
If you use the formula (P0 – PS) / PS = nB / nA, the molarity is nearly same as above (2.7 M).
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