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(III) chloride, which would react with silver nitrate solution like this:

FeCl3(aq)+3AgNO3(aq)3AgCl(s)+FeNO33(aq)

The chemist adds 81.0mM silver nitrate solution to the sample until silver chloride stops forming. He then washes, dries, and weighs the precipitate. He finds he has collected 8.2mg of silver chloride. Calculate the concentration of iron(III) chloride contaminant in the original groundwater sample (in mg/L). Round your answer to 2 significant digits.

Explanation / Answer

m = 8.2 mg of AgCl

mol = mass/MW = (8.2*10^-3)/(143.32 ) = 0.00005721 mol of AgCl

then

mol of Cl = 0.00017163

mol of FeCl3 = 1/3*0.00017163= 0.00005721 mol of FeCl3

[FeCl3] = 0.00005721 /V

V = 200 mL = 0.2 L

[FeCl3] = 0.00005721/0.2 = 0.00028605 M --> 2.9*10^-4 M of FeCl3

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