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 cadmium chloride, which would react with silver nitrate solution like this: CdCl_2(aq) + 2 AgNO_3 (aq) rightarrow 2AgCl(s) + Cd(NO_3)_2 (aq) The chemist adds 34.0 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 3.1 mg of silver chloride. Calculate the concentration of cadmium chloride contaminant in the original groundwater sample. Round your answer to 2 significant digits.

Explanation / Answer

[AgNO3] = 34 mM = 34*10^-3 M

AgCl(s) forms...

m = 3.1 mg of AgCl

find concentratino of CdCl in water...

so..

mass of AgCl = 3.1 mg = 3.1*10^-3 g

mol = mass/MW = (3.1*10^-3)/143.32 = 0.00002162 mol of AgCl

relate mol of Ag+ with mol of AgNO3 = 0.00002162

mol of AgNO3 = 0.00002162

ratio with respect to CdCl2:

1:2

0.00002162 mol --> 1/2*0.00002162 = 0.00001081 mol of CdCl2

so

mass of CdCl2 = mol*MW = 0.00001081*183.32 = 0.00198 g of CdCl2

mg = 0.00198*10^3 mg = 1.98 mg of CdCl2

Concentration = mass / Volume = 1.98 /250 = 0.00792 mg /L

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