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 tin(II) chloride, which would react with silver nitrate solution like this: SnCl2 (aq) + 2AgNO3 (aq) 2AgCl (s) + SnNO32 (aq) The chemist adds 31.0m 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 5.3mg 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

Reaction that would happen is

SnCl2 (aq) + 2 AgNO3 (aq) ------> 2 AgCl (s) + Sn(NO3)2 (aq)

1 mole 2 mole 2 mole 1 mole

Given 5.3 mg (0.053 g) of AgCl is obtained

Molar mass of AgCl = 143.32 g/mol

No. of moles of AgCl = Mass / Molar mass = 0.053 g / 143.32 g/mol = 3.7 * 10-4 moles of AgCl

Two moles of AgCl preciptate is formed per mole of SnCl2 reacting according to reaction stoichometry

so 3.7 * 10-4 moles of AgCl will be formed when (3.7 * 10-4 / 2 =) 1.85 * 10-4 moles of SnCl2

1.85 * 10-4 moles of SnCl2 will be present in given 250ml(=0.25 L) of groundwater

Concentration of SnCl2 = 1.85 * 10-4 moles of SnCl2 / 0.25 L = 7.4 * 10-4 mol/L (M) Answer

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