One way the U.S. Environmental Protection Agency (EPA) tests for chloride contam

Question

One way the U.S. Environmental Protection Agency (EPA) tests for chloride contaminants in water is by titrating a sample of silver nitrate solution. Any chloride anions in solution will combine with the silver cations to produce bright white silver chloride precipitate. Suppose an EPA chemist tests a 200. mL sample of groundwater known to be contaminated with iron(II) chloride, which would react with silver nitrate solution like this: feCl_2(aq) + 2 AgNO_3 (aq) rightarrow 2 AgCl(s) + Fe(NO_3)_2(aq) The chemist adds 48.0 mM silver nitrate solution to the sample until silver chloride stops forming, she then washes, dries, and weighs the precipitate. She finds she has collected 8.5 mg of silver chloride. calculate the concentration of iron(II) chloride contaminant in the original groundwater sample. Be sure your answer has the correct number of significant digits.

Explanation / Answer

we have

mass of AgCl = 8.5 mg

and

volume of solution = 200 ml = 0.2 L

and

moles of AgCl = mass / molar mass = 8.5 mg / 143.32 g/mol = 5.93 x 10^-5 mol

therefore

moles of FeCl2 = moles of AgCl / 2

= 5.93 x 10^-5 mol / 2

= 2.97 x 10^-5 mol

and

mass of FeCl2 = 126.751 g/mol x 2.97 x 10^-5 mol = 3.76 mg

and

molarity of FeCl2 = mass / volume

= 3.76 mg  / 0.200 L

= 18.8 mg / L

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