chemical energetics?

Question

the lattice energies of rubudium fluoride RbF and caseium chloride,CsCl are -760kj/mol and -650kj/mol respectively.what is lattice energy of caesium fluoride CsF likely to be
option are
a:-620kj/mol
b:-720kj/mol
c:-800kj/mol
d:-900kj/mol
its answer is b,plz explain me how we will do this
thanks

Answer 1

according to coulombs law= K.Q1*Q2/d

K=constant=8.99.10sub9J-m/c2
use the Cscl and Rbf energies to calculate the distance between the atoms once calculated determine the lattice energies using the distance and potential energies.