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Question

Dry air was bubbled through a bulb containing 22 g of an non-volatile organic compound in 150 g of water, then it passed through a bulb containing water at the same temperature and finally through a bulb containing anhydrous calcium chloride. The weight loss in bulb containing water was 0.075 g
and gain in mass of the calcium chloride tube was 1.5 g.
Calculate the molecular mass of the organic substance.

A
50.16 g/mol
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B
59.16 g/mol
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C
42.16 g/mol
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D
64.16 g/mol
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Solution

The correct option is A 50.16 g/mol
Given,
Gain in mass of the CaCl2=1.5 g
Loss in mass of the pure solvent =0.075 g
By Ostwald-Walker method,

We have,

ppsp=Loss in mass of solvent bulbGain in mass of CaCl2 tube
ppsp=0.0751.5
For a non-volatile solute,
ps=p×Xsolventpp.Xsolventp=0.0751.51Xsolvent1=0.0751.5Xsolute1=0.0751.5nsolutentotal=0.0751.5
where,
nsolute is the number of moles of solute
ntotal is the number of moles of solute and solvent.
Let the molecular mass of the organic substance be 'm'
thus,
nsolutensolute+nsolvent=w/mwm+WM
Where,
M is molar mass of water
W is mass of solvent
w is mass of solute
m is molar mass of solute

0.0751.5=22m22m+150182222+15018m0.0751.5×[22+15018m]=22m=50.16 g/mol


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