What is the distance between two spheres, each with a charge of 2.5 x 10-6 c, when the force between them is 0.50 n?

In this case, Coulomb's Law applies:

F = 1 / (4πε₀) · (Q₁Q₂/r²)

You can solve it for r:

r = √[1 / (4πε₀) · (Q₁Q₂/F) ]

Plugging in numbers:

r = √[1 / (4π·8.85*10⁻¹²) · (2.5*10⁻⁶) ²/0.50]

= 0.335m

The correct answer is: the two charges are 0.335m apart.