Question

The membrane that surrounds a certain type of living cell has a surface area of 4.50 × 10-9 m2 and a thickness of 1.00 × 10-8 m. Assume that the membrane behaves like a parallel plate capacitor and has a dielectric constant of 5.3. The potential on the outer surface of the membrane is +62.0 mV greater than that on the inside surface. If the charge is due to positive ions (charge +e), how many such ions are present on the outer surface?

Answer 1

q = kappa epsilon A V/d
q = 5.3* 8.8e-12 * 4.5e-9 *62e-3 / 1e-8

Once you have this number the charge of a single ion (1.67e-19) to find out how many charges you need.