Question

The concentration of the alkane C₂₀H₄₂ (FM 282.55) in a particular sample of rainwater is 0.18 ppb. Assume that the density of rainwater is close to 1.00 g/mL and find the molar concentration of C₂₀H₄₂.

Answer 1

Molar concentration is concentration in moles /liter. This is also referred to as molarity, which is the most common method of expressing the concentration of a solute in a solution.
Given that the density of rain water = 1.00 g/mL

M = 282.55

Concentration of alkane in a sample of rain water = 0.18 ppb = 0.18 x 10^-9 g

In one liter, 1000g of rainwater is present.

The mass of C₂₀H₄₂ is 0.18 x 10^-9 x 10³ = 0.18 x 10^-6 g

Molar concentration of given alkane = (0.18 x 10^-6 )/282.55 = 6.38 x 10^-10 M