Suppose that a device is using a 5.00-mg sample of silicon that is doped with 1 × 10-5% (by mass) phosphorus. How many phosphorus atoms are in the sample?

Answer 1

For who needs it in the future:
5.00mg Si / 1000 mg Si = 0.005g Si

(0.005)((1e-5)/100g Si) = 5.00e-10

((5.00e-10)(6.02e23))/(31 g P) = 9.71e12 atoms P

Answer 2

P = 5.00 mg * 1E-5 % = 5.00E-10 g

P atoms = (5.00E-10 / 31.0) * 6.02E23