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 75.0m M silver nitrate solution to the sample until silver chloride stops forming. She then washes, dries, and weighs the precipitate. She finds she has collected 6.6mg of silver chloride. Calculate the concentration of iron(III) chloride contaminant in the original groundwater sample. Round your answer to 2 significant digits.

Explanation / Answer

FeCl3 (aq) + 3 AgNO3 (aq) --> 3 AgCl (s) + Fe(NO3)3 (aq)

AgCl collected = 6.6 mg = 0.0066 g   ( 1mg = 0.001 g)

AgCl moles = mass / Molar mass of AgCl = 0.0066 g / 143.32g/mol   = 0.00004605

Moles of FeCl3   = ( 1/3) AgCl moles               ( since 3 AgCl formed from 1FeCl3)

             = ( 1/3) x 0.00004605 = 0.00001535

Mass of FeCl3 = moles x molar mass of FeCl3

       = 0.00001535 mol x 162.2 g/mol    = 0.0025 g = 2.5 mg

volume of solution used = 200 ml = 0.2 L

concentration of FeCl3 = 2.5 mg / 0.2 L = 12 mg /L

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