the molar concentration is concentration in moles /Liter 1kg=1000g in a ltr. of water mass, So by assuming that the density of rainwater is close to 1.00 g/ml.the concentration of C20H42 (fm 282.55) in a particular sample of rainwater is 0.14 ppb is 0.32*10^-9*10^3= 0.32*10^-6g/L now, to have this in mole divide by molar weight 0.32*10^-6/282.55=1.133*10^-9mole/Liter.
finally
So answer 1.133*10^-9M
