Question

how to find vant hoff factor for weak acids .....

Answer 1

In solution some compounds get dissociate and some get associate due to which no. Of mole of that compound changes and essentially colligative properties depend upon no. Of moles so that vary too…
Now for glucose that neither dissociate nor associate, it simply gets solvated, so No. Of moles remain constant and colligative properties also…
i=Observed colligative properties/Calculated colligative properties
For glucose both numerator and denominator are same that’s why i=1..
For MgSo4, It dissociate into two ion Mg2+and Sulphate ion.
So its No. Of mole increases by two that means colligative property observed also multiply by 2 and therefore we get:
i=2 for MgSo4In general dissociation leads to i>1, association leads to i<1 and i=1 for neither association nor dissociation (Glucose)Now for the cases like neither full dissociation nor association, extent of dissociation or association is considered :
Ex: Acetic acid, weak acid dissociate weekly.. CH3COOH=(CH3COO-) +(H+)Let Extent of dissociation is x. If we started with n moles of acetic acid then after dissociation CH3COOH=n(1-x), (CH3COO-)=(H+) =n(x)So total mole =n(1+x)i=observed colligative properties /calculated colligative properties…., and colligative properties are depends upon no. Of moles so we can say:i=observed no. Of moles /Calculated no. Of molesThat’s why in case of weak dissociation we get i=n(1+x)/n that gives i=1+x…i>1