O CHEMICAL REACTIONS -Solving for a reactant in solution One way the U.S. Enviro

Question

O CHEMICAL REACTIONS -Solving for a reactant in solution 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(II) chloride, which would react with silver nitrate solution like this: FeCl 2(aq) + 2 AgNO3(aq) 2 AgCl (s) + Fe(NO3) 2(aq) The chemist adds 77.0 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 8.1 mg of silver chloride. Calculate the concentration of iron(II) chloride contaminant in the original groundwater sample. Round your answer to 2 significant digits. mg -10

Explanation / Answer

mass of AgCl obtained as precipitate=8.1 mg=0.0081g

mol of AgCl=mass /molar mass=0.0081g/143.32g/mol=0.00005652=5.652*10^-5 mol

mol of Cl-(chloride ) in the sample=mol of AgCl=5.652*10^-5 mol(present in 1:1 molar ratio)

As FeCl2 has Cl- and Fe2+ in 1:2 molar ratio,2 mol of Cl- per mol FeCl2 ,so mol of FeCl2 =2*mol of Cl-(chloride ) in the sample

or,mol of FeCl2 =2*5.652*10^-5 mol=1.130*10^-4mol

Concentration of FeCl2 in the sample=mol/volume=1.130*10^-4mol/0.250L=4.521*10^-4 mol/L

mass of FeCl2=4.521*10^-4 mol/L*molar mass of FeCl2=4.521*10^-4 mol/L*126.751 g/mol=0.05730 g/L=0.05730*1000 mg/L=5.730*10^1 mg/L

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