Question

if the enthalpy of formation and enthalpy of solution of HCl(g) are -92.3 kJ/mol and -75.14 kJ/mol respectively then find enthalpy of formation of Cl-(aq)

Answer 1

well first of all you need to know the reaction
HCL(g)+aq ------>H+(aq)+Cl-(aq)
here enthalpy of solution given is -75.14
there fore the sum enthalpy of solution and enthalpy of HCl should be equal to sum final enthalpy of H+ and even final enthalpy of CL- ion
well note that we are talking about change in enthalpy because we cannot determine the the absolute value of enthalpy 
so
-75.14+ enthalpy of HCl(g)= (final enthalpy of h+ ) and (final enthalpy of Cl-)
but enthalpy of H+ ion is 0
therefore
enthalpy of CL- = -75.14-92.3=-167.44 kJ/mol
Hope this helped you 
:-)