The drinking water in Saplingville was found to contain 14 ppb (parts per billion) of lead. What is the concentration of lead in molarity? concentration of lead = M

Answer 1

Before you begin, make sure you look up the molar mass of lead (Pb), which I have as 207.2 g/mol. Use the molar mass in your textbook or the periodic table your instructor provided you.

(1) Conceptualize what is going on.

It is often easier to think of these problems in terms of dimensional analysis and by thinking that this is just a change of units questions.

We have 14 ppb lead, which means 14 g Pb / 1 billion g H2O.

We have units g Pb / g H2O, but we need mol Pb / L H2O (remember that molarity is defined as mol/L).

(2) Convert grams of lead to moles of lead via molar mass.

14 g Pb * (1 mol Pb / 207.2 g Pb) = 0.067567 moles Pb (I am ignoring sig figs for now)

(3) Convert g H2O to L H2O via density.

Remember that the density of water is 1 g/mL.

10⁹ g H2O x (1 mL / 1 g) x (1 L / 10³ mL) = 1,000,000 L H2O (again, ignoring sig figs for now)

(4) Calculate molarity

[Pb] = mols Pb / L H2O = 0.067567 moles Pb / 1,000,000 L H2O = 6.8 x 10^-8 M Pb (two sig figs, limited by the original 14 ppb)