Question

14. The EPA maximum safe limit for arsenic exposure is 50 ppb. If a 25 mL water sample was found to contain 50 ng of arsenic, would this water be deemed safe? Assume the density of water to be 1 g/mL.

Answer 1

We have formula , ppb = [ Mass of solute/ Mass of solution ] 10 ⁹

Mass of As = 50 ng = 5 $\times$ 10 ^-08 g

We have denstiy = Mass / volume

Therefore, Mass = density $\times$ volume

Therefore, Mass of solution = mass of water = 1 g /ml $\times$ 25 ml = 25 g

$\therefore$ ppb As = ( 5 $\times$ 10 ^-08 g / 25 g ) 10 ⁹ = 2 ppb

Concentration of As in water sample is 2 ppb and safe limit is 50 ppb. Hence , we can assume this water is safe.