The lattice energy of a potassium chloride crystal containing N molecules of KCl is given by:
U = N(αq²/r - b/rⁿ

Determine the repulsive exponent using the following data:
Nearest neighbor equilibrium distance r = -3.15 Ã….
Compressibility of KCl, K = 5.8 x 10^11 m^2/N.
Madelung constant,α = 1.75.