Question

A nonvolatile organic compound Z was used to make up a solution. Solution A contains 5.00 g of Z dissolved in 100 g of water and has a vapor pressure of 754.5 mmHg at the normal boiling point of water. Calculate the molar mass of Z in solution A.

Step by Step solution please

Answer 1

Mass of water = 100 g

The mass of solute = 5g

no of moles of water   = W/G. M.Wt

                                   = 100/18 = 5.56mole

The vapor pressure of water at 100⁰C   = 760mmHg

The vapor pressure of solution at 100⁰C   = 754.5 mmHg

According to roults law

vapor pressure of solution = mole fraction of water *P⁰

mole fraction of water = vapor pressure of solution/P⁰

                                     = 754.5/760 = 0.992

mole fraction of solute   = 1-0.992 = 0.008

mole fraction of solute = n1/n1 + n2

                n1/n1 + n2       =   0.008

                n1+n2/n1       = 1/0.008

                n1+ 5.56/n1   = 125

               n1 + 5.56         = 125n1

                  5.56           = 125n1-n1

                 5.56            = 124n1

                   n1            = 5.56/124 = 0.045

molar mass of solute Z    = mass of solute Z/no of moles of solute

                                         = 5/0.045   = 111g/mole