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2.36   100\; g of liquid A (molar\; mass \; 140\; g\; mol^{-1}) was dissolved in 1000\; g of liquid B (molar\; mass\; 180\; g\; mol^{-1}). The vapour pressure of pure liquid B was found to be 500 \; Torr.Calculate the vapour pressure of pure liquid A and its vapour pressure in the solution if the total vapour pressure the solution is  475 \; Torr.

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For calculating partial vapour pressure we need to calculate mole fractions of components.

So number of moles of liquid A :

                                                        = \frac{100}{140} = 0.714

and  moles of liquid B :

                                                       = \frac{1000}{180} = 5.556

Mole fraction of A (xA)   :

                                                      = \frac{0.714}{0.714+5.556} = 0.114

and mole fraction of B (xB)  : 

                                                      = \frac{5.556}{0.714+5.556} = 0.866

Now,                    Ptotal  =  PA + PB

or                         P_{total} = P_A^{\circ}x_A\ + P_B^{\circ}x_B

or                          475 = P_A^{\circ}\times0.114\ + 500\times0.886

or                            P_A^{\circ} = 280.7\ torr

Thus vapour pressure in solution due to A =   P_A^{\circ}x_A                            

                                                                    = 280.7 \times0.114 = 32\ torr

Posted by

Devendra Khairwa

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