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 250. mL sample of groundwater known to be contaminated with iron(III) chloride, which would react with silver nitrate solution like this: Fecisfaq) + 3 AgNO3(aq) 3 AgCI(s) + Fe(NO3)3(a) The chemist adds 58.0 mM silver nitrate solution to the sample until silver chloride stops forming. He then washes, dries, and weighs the precipitate. He finds he has collected 5.2 mg of silver chloride. Calculate the concentration of iron(III) chloride contaminant in the original groundwater sample. Round your answer to 2 significant digits. mg

Explanation / Answer

[AgNO3] = 58*10^-3 M

m = 5.2 mg of AgCl

find mol of AgCl

mol = mass / MW = 5.2/143.32 = 0.036282 mmol

now..

mmol of Cl- =  0.036282 mmol

mmol of Fe+3 = 3*mmol of Cl-

mmol of Fe+3= 0.036282/3 = 0.012094 mmol of Fe+3

1mmol of Fe+3 = 1 mmol of FECl3

mmol of Fe(Cl)3 = 0.012094

mass = mmol*MW = 0.012094*162.2 = 1.961 mg per 250 mL

1.961/0.25 = 7.8 mg of FeCl3 / Liter

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