Question

1. A sample of lake water is determined to have a benzene concentration of 0.1 µmol/L. Is this concentration above or below the U.S. EPA maximum contaminant level (MCL) for benzene? Prove your answer with calculations.

Answer 1

The value of U.S. EPA maximum contaminant level (MCL) for benzene is 0.005 mg/L. Now convert the concentration of benzene (0.1 umol/L) in the sample of a lake water into mg/L Molecular formula of benzene , C6H6=78 g/mol Amount in g = number of moles * molar rmass Concentration of benzene in lake water sample = 0.1 umol/L = 0.1 umol * (78 g/mol)/L = 7.8 ug/L since we know that 1 ug = 0.001 mg, therefore Concentration of benzene in lake water sample = 7.8 * 0.001 mg/L = 0.0078 mg/L Concentration of benzene in lake water sample=0.0078 mg/L Hence the concentration of benzene in lake water sample is above the U.S. EPA maximum contaminant level (MCL) for benzene (which is 0.005 mg/L).