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