Question

The maximum allowable level of selenium in drinking water is 50ppb. A chemist found 34ug of selenium in 500mL of lake water. Would you drink that lake water? Why or why not?

Answer 1

The maximum allowable level of selenium in drinking water is 50 ppb, i.e. 50 molecules of selenium per 10⁹ molecules of water.

In 34 ug of Se there is 34*10^-6 g / 79 g/mole = 4.3*10^-7 mole of Se, or 4.3*10^-7*6.02*10²³ = 2.59*10¹⁷ molecules of selenium.

In 500 mL of water (density = 1 g/mL) there is 500 g / 18 g/mole = 27.78 mole of H2O, or 27.78*6.02*10²³ = 1.67*10²⁵ molecules.

The relative amounts of molecules of Se : H2O = 2.59*10¹⁷ : 1.67*10²⁵ = 1.55*10¹⁷ : 10²⁵ = 15.5 : 10⁹ , i.e. 15.5 ppb of selenium. The water is drinkable, since selenium content is below the allowable limit.