AP EAMCET Engineering 23 Apr 2019 Shift 1 Paper

The number of moles of solute (n1) will be 7.5/M if we take the molar mass of the solute to be M g/mol.
The moles of solvent (n2) will be 90/18 = 5 moles
The partial pressure comes out to be,
p=

P0−Pa

P0

=

P0−2.8

P0

=

7.5∕M

5

P0−2.8

P0

=

1.5

M2

…… (i)
Now, when 18 gram of water is added then

P0−2.81

P0

=

7.5∕M

108∕18

=

1.25

M

…… (ii)
On solving equation (i) and (ii) we get,
M = 71.5 g/mol