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 sample of groundwater known to be contaminated with copper(II) chloride, which would react with silver nitrate solution like this:

(aq) (aq) (s) (aq)

The chemist adds M silver nitrate solution to the sample until silver chloride stops forming. He then washes, dries, and weighs the precipitate. He finds he has collected of silver chloride. Calculate the concentration of copper(II) chloride contaminant in the original groundwater sample. Round your answer to significant digits.

Explanation / Answer

The reaction is:

CuCl2(aq) + 2AgNO3(aq) ------- > 2AgCl(s) + Cu(NO3)2(aq)

Mass of AgCl collected = 3.3 mg = 0.0033 gm

Now, number of moles of AgCl = mass / molar mass of AgCl

= 0.0033 gm/ 143.32 gm/mol

= 0.000023 moles

From above chemical equation,

1 mol CuCl2 gives 2 mol AgCl

So, 0.000023 mol AgCl * 1 mol CuCl2 / 2 mol AgCl = 0.0000115 mol CuCl2

Now, Molarity = moles of CuCl2 / volume in liter

= 0.0000115 mol / 200mL

=  0.0000115 mol / 0.2 L

                    = 0.0000576 M

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