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 tin(II) chloride, which would react with silver nitrate solution like this: SnCl2 (aq) + 2AgNO3 (aq) 2AgCl (s) + SnNO32 (aq) The chemist adds 58.0m 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 4.0mg of silver chloride. Calculate the concentration of tin(II) chloride contaminant in the original groundwater sample. Round your answer to 2 significant digits.

Explanation / Answer

We know that moles of AgCl=2*moles of SnCl2 according to the reaction hence we know that 4mg AgCl is collected hence moles of AgCl=(4*10^-3)/143.32=2.79*10^-5 moles

Hence moles of SnCl2=2.79*10^-5/2=1.39*10^-5

Hence concentration of SnCl2=1.39*10^-5/0.2=6.97*10^-5 M

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