R-COOH dimerises in benzene upto 50% and it ionises upto 10% in water.Detrmine the ratio of Van't hoff factors in dimerisation v/s ionisation?
Here, we know that , degree of dissociation α = no. of moles associated / total no. of moles taken.
If we take 1 mole of RCOOH in benzene , we get 50 % of it dimerised so, no of moles associated = 1/2
Hence, α = 1/2
For dimerisation of 1 mole of RCOOH, we need to take 2 molecules of it so no. of molecules , n= 2
The reaction is :
................2RCOOH ⇔ (RCOOH)2
Before asso. 1 mole......... 0
after asso. 1-α ...................α /2
Now, we know that, i = 1-α + α /n
Here, n = 2
So, van't hoff factor i = 1-α + α /2= 1-1/2 + (1/2)/2 = 3/4
Now, in case of dissociation
................RCOOH ⇔ RCOO- + H+
Before asso. 1 mole......... 0....................0
after asso. 1-α ...................α ..... ......α
Van't hoff's factor i = 1+ (n-1)α/1
Here, n = 1 so, i = 1+ α
Now, we know that dissociation is 10% , so for 1 mole, α = 1/10
So, i = 1+ 1/10 = 11/10
hence,ratio of , i of dimerisation/ i of ionization = (3/4 )/ (11/10) = 15/22