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:

FeCl2(aq) + 2AgNO3(aq) ---> 2AgCl(s) + Fe(NO3)2(aq)

The chemist adds 83.0 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 8.3 mg of silver chloride.

Calculate the concentration of iron(II) chloride contaminant in the original groundwater sample. Round your answer to 2 significant digits.

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Explanation / Answer

mass of AgCl= 8.3 mg, molar mass of AgCl= 143.5 g/mole, moles of AgCl= mass/molar mass = 8.3*10-3 gm/143.5 =5.8*10-5 moles

The reaction is FeCl2+ 2AgNO3 ---------->Fe(NO3)2+2AgCl(s)

2 moles of AgCl requires 1 mole of FeCl2 and 2 moles of AgNO3.

moles of FeCl2 = 5.8*10-5 moles/2= 2.9*10-5 moles

volume of solution = 200ml =0.2L (1000ml=1L)

concentration of FeCl2= moles/Volume in L = 2.9*10-5/0.2 =1.45*10-4 M

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