Question

(a) What type of deviation is shown by a mixture of ethanol and acetone? Give reason.

(b) A solution of glucose (molar mass =$180gmo{l}^{-1}$) in water is labelled as 10% (by mass). What would be the molality and molarity of the solution? $(Densityofsolution=1.2gm{L}^{-1}$)

Answer 1

(a) Positive deviation is shown by a mixture of ethanol and acetone. In pure ethanol, a very high fraction of molecules are hydrogen bonded. On adding acetone, its molecules, get in between the molecules of ethanol, thus breaking the hydrogen bonds and reducing ethanol-ethanol attractions considerably.

As a result escaping tendency of molecules into the vapour phase increases. Partial vapour pressure of each component and total vapour pressure of the solution comes out to be more than that expected from Raoult's law. Hence, this solution shows positive deviation.

$\dots H-\underset{C_2{H}_5}{\underset{|}{O}}\dots H-\underset{C_2{H}_5}{\underset{|}{O}}\dots H-\underset{C_2{H}_5}{\underset{|}{O}}\dots$

(b) Mass of glucose = 10 g

Mass of solution = 100 g

Mass of solvent = 100 - 10 = 90 g

Mol. mass of glucose = $18gmo{l}^{-1}$

Vol. of solution $=\frac{Mass}{Density}=\frac{100}{1.2}$

$=\frac{1000}{12}=83.33c{m}^3$

$\text{Molarity}=\frac{10}{180}\times \frac{1000}{83.3}=0.67M$

$\text{Molality}=\frac{10}{180}\times \frac{1000}{90}=0.62m$