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 copper(II) chloride, which would react with silver nitrate solution like this: CuCl2 (aq) + 2AgNO3 (aq) → 2AgCl (s) + CuNO32 (aq) The chemist adds 26.0m 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 6.1mg of silver chloride. Calculate the concentration of copper(II) chloride contaminant in the original groundwater sample. Be sure your answer has the correct number of significant digits.

Answer 1

The balanced chemical reaction is expressed as follows:

CuCl2 (aq) + 2AgNO3 (aq) → 2AgCl (s) + CuNO32 (aq)

To determine the concentration of copper(II) chloride contaminant in the original groundwater sample, we use the final amount of silver chloride that was produced from the reaction and the relation of the substances from the chemical reaction. We calculate as follows:

mmol AgCl = 6.1 mg AgCl ( 1 mmol / 143.35 mg ) = 0.0426 mmol
mmol CuCl2 = 0.0426 mmol AgCl ( 1 mmol CuCl2 / 2 mmol AgCl ) = 0.0213 mmol CuCl2
concentration of CuCl2 in the original water sample = 0.0213 mmol CuCl2 / 200.0 mL = 1.0638 x 10^-4 mmol / mL or 1.0638 x 10^-4 mol/L