If the freezing point of 0.1M HA(aq) solution is -0.2046°C then pH of the solution is? (Kf of water=1.86/mol kg)

1) 1 2)1.3 3)1.7 4)2

Dear student,
_pH varies widely with concentration of the solution, if the concentration of acid is very high like in this case [0.1M], the H⁺ions concentration of just the acid HA will be considered and contribution of water to H⁺ions will be ignored.
pH = -log[H⁺] only from acid in this case and not water since concentrated solution.
pH = -log[0.1] =-log[10^-1]= -(-1)= 1.