Question

The concentration of the alkane C₂₀H₄₂ (FM 282.56) in a particular sample of rainwater is 0.2 ppb. Assume that the desnity of rainwater is close to 1.00 g/mL and find the molar concentration of C₂₀H₄₂.

Answer 1

1 ppb = 0.001 ppm

0.2 ppb = 0.2 x 0.001 ppm = 0.0002 ppm

ppm = mg / L

0.0002 ppm = 0.0002 mg / L

                    = 2 x 10^-7 g / L

                   = 2 x 10^-7 / 282.56 (mol / L)

                   = 7.08 x 10^-10 mol / L

molar concentration = 7.08 x 10^-10 M