Solving for a reactant in solution

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 iron(III) chloride, which would react with silver nitrate solution like this: FeCl3(aq) + 3AgNO3(aq) → 3AgCl(s) + FeNO33(aq) The chemist adds 60.0mM silver nitrate solution to the sample until silver chloride stops forming. He then washes, dries, and weighs the precipitate. He finds he has collected 5.7mg of silver chloride.Calculate the concentration of iron(III) chloride contaminant in the original groundwater sample. Be sure your answer has the correct number of significant digits.

Answer 1

FeCl3(aq) + 3AgNO3(aq) → 3AgCl(s) + FeNO33(aq)

Molar mass AgCl = 143 g/mole

moles AgCl formed = 5.7 mg x 1g/1000 mg x 1 mol/143 g = 3.99x10^-5 mols AgCl

moles FeCl3 = 3.99x10^-5 mols AgCl x 1 mol FeCl3 / 3 moles AgCl = 1.33x10^-5 mols

Molar mass FeCl3 = 162 g/mol

grams FeCl3 = 1.33x10^-5 mols FeCl3 x 162 g/mol = 0.00215 g FeCl3 in 250 ml (0.250 L) sample

Concentration FeCl3 = 0.00215 mol/0.250 L = 0.00862 M (3 sig digits)