The vapour pressure of acetone at ( 20^{circ} mathrm{C} ) is 185 torr. When ( 1.2 mathrm{g} ) of a non-volatile substance was dissolved in ( 100 mathrm{g} ) of acetone at ( 20^{circ} mathrm{C}, ) its vapour pressure was 183 Torr. The molar mass of the substance is
(a) 32
(b) 64
(a) 128
(b) 488

Given, ( p^{circ}=185 ) Torr at ( 20^{circ} mathrm{C} )
[
p_{s}=183 text { Torr at } 20^{circ} mathrm{C}
]
Mass of non-volatile substance, ( m=1.2 mathrm{g} ) Mass of acetone taken ( =100 mathrm{g} )
[
M=?
]
As, we have ( frac{p^{circ}-p_{s}}{p_{s}}=frac{n}{N} )
Putting the values, we get, ( frac{185-183}{183}=frac{frac{1.2}{11}}{frac{100}{58}} )
( Rightarrow quad frac{2}{183}=frac{1.2 times 58}{100 times M} )
( therefore quad M=frac{183 times 1.2 times 58}{2 times 100} )
( M=63.684=64 mathrm{g} / mathrm{m} Omega )