ionic bonding question

Question

Confused with question: Calculate the force of attraction (in N) between a cation with a valence of +1 and an anion with a valence of -3, the centers of which are separated by a distance of 9.8 nm?

My answer is 7.05E-20

I am not sure whether it's supposed to be negative or not because of the anion or where I went wrong.

I am also given this formula for the problem hints: (2.31× 10^−28 N⋅m2)(|𝑍1|)(|𝑍2|)/𝑟^2

The equation i used : (9x10^9)(+1 x 1.6x10^-19)(-3x 1.6x10^-19)/(9.8x10^-9). which is the proper formula?

Answer 1

I'm pretty sure you are using Coulomb's law, so let me see if I get the same value that you get.

Coulomb's law:

F = (9x10^-9 ) * |(+1 * 1.6x10^-19) (-3*1.6x10^-19)| / (9.8x10^-9 )²

F = 1.44x10^-27 * 4.8x10^-19 / 9.6x10^-17'

F = 7.2x10^-30 N

I get a different value, but I notice that you did not divide by the radius squared, just divided by the radius.