Fluorine is more electronegative than chlorine even then, p-flurobenzoic acid is weaker acid than p-chlorobenzoic acid explain ?

Since halogens are more electronegative than carbon and also possesses lone pair electrons , therefore they exert both -I and +R effects . Now in F , the lone pair of electrons are present in 2p-orbitals but in Cl , they are present in 3p-orbitals . Since 2p- orbitals of fluorine and carbon are almost  equal size , therefore , the +R effect is more pronounced in p- flurobenzoic acid than p-chlorobenzoic acid.

 
Thus in p- flurobenzoic acid +R effect is out weight the -I effect but in p-chlorobenzoic acid, it is the -I effect which is out weight  the +R effect. Hence p- flurobenzoic acid is weaker acid than p-chlorobenzoic acid.

Trifluoroethyl carbocation is less stable than trifluromethyl carbocation why ?

In case of trifluroethylcarbocation the highly electronegative fluorine atom , withdraws shared pair electrons between C-F bonds towards itself to great extent and intensifies positive charge on carbon. Greater the intensification of charge more is the instability.theirfore this carbocation is destabilised.

In case of trifluroethylcarbocation carbocation, the unshared paired of electrons in P - Orbital of each of the fluorine can be shifted into vacant Orbital of carbon atom of carbocation via P-P overlap.

It's leads to dispersal of charge and provide stability to the carbonation.