Raoult's Law for ideal solution, Graphical representation of Vapour Pressure and mole fraction of volatile components in ideal solutions, Chemistry class XII, Chapter 2- Solutions
P0A is not equal to P0B being the maximum vapour pressure of two different liquids A & B. In the example P0B>P0A.Hence total Vapour pressure of solution Ptotal , the line joining P0A and P0B, will not be a straight line. I hope now it is clear.
This graph variation of Vapour Pressure Pa and Pb with their mole fractions in the solution. At initial point Mole fraction of A is maximum i.e.1. Xa=1 whereas Xb=0. but going from left to right Xa decreases and becomes 0 at extreme right. Left to Right Xb increases from 0 to 1. So the graph has covered all the mole fractions (concentrations ) of both the liquids. At any point on the x-axis the Ptotal is determined by the red line. Pt=Pa+Pb. So total pressure can be determined at any concentrations of two liquids. I hope that the point is clear now. The graph starts with P0a(Xa=1 and Xb=0)and ends at P0b (Xa=0 and Xb=1) and in between the two points covers all the mole fractions of the two liquids A and B.
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The graph is little bit difficult in view of the fact that it is 3 in 1 graph. It shows change in vapour pressures of both the volatile parts A and B with respect to their mole fractions in the Solution. The blue line shows the values of vapour pressure of solute A ( along Y axis at left side) with respect to mole fraction of A ( along common X-axis). Mole Fraction of A increases from Right to Left. At the pont of right Y axis it is 0 and at at the point of left Y axis it is maximum=1. Vapour pressure decreases with decrease in Mole Fraction . It is maximum when Mole fraction is maximum=1 and Vapour pressure =0 when mole fraction=0. Similarly for volatile part B the green line shows change in Vapour Pressure of B ( Along Y axis at Right side) with respect to mole fraction B ( along common X-axis) . Mole fraction of B increases from left to right along common X axis. The red line shows the total Vapour pressure of solution by adding both vapour pressures due to A ( Blue Line) + vapour pressure due to B ( green Line). After reading this explanation please watch the graphical part of the video again. I hope you will have no doubts. Thanks
