Finding the concenration of a substance?

Question

Hydrogen sulfide can be deteced at a concentration of 2.00ppb. The density of air is 1.2929 g/L

1)What is the concentation of H2S in mg/L at the detection level?

2) The lethal dose of hydrogen sulfide by inhalation for rats is 7.13×10−4g/L . What is this lethal dose in ppm?

Answer 1

ppb means part-per-billion, and ppm means part-per-million.

1) 1.2929 g/L * 2.00x10^-9 = 2.5858x10^-6 mg/L

2) (7.13×10−4g/L) / (1.2929 g/L) = 5.51×10−4 = 551 ppm