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 Vke this: SoCl(aq) + 2 AgNO3(aq) + 2 AgCl(s) + Sn(NO), (aa) ob The chemist adds 30.0 mm silver nitrate solution to the sample until silver chloride stops forming. She then washes, dries, and weighs the precipitate. She finds she has collected 9.0 mg of silver chloride. Calculate the concentration of tin(II) chloride contaminant in the original groundwater sample. Be sure your answer has the correct number of significant digits.

Answer 1

Molar mass of AgCl,
MM = 1*MM(Ag) + 1*MM(Cl)
= 1*107.9 + 1*35.45
= 143.35 g/mol

mass of AgCl = 9*10^-3 g
mol of AgCl = (mass)/(molar mass)
= 9*10^-3/1.434*10^2
= 6.278*10^-5 mol


According to balanced equation
mol of SnCl2 reacted = (1/2)* moles of AgCl
= (1/2)*6.278*10^-5
= 3.139*10^-5 mol


Molar mass of SnCl2,
MM = 1*MM(Sn) + 2*MM(Cl)
= 1*118.7 + 2*35.45
= 189.6 g/mol


mass of SnCl2 = number of mol * molar mass
= 3.139*10^-5*1.896*10^2
= 5.952*10^-3 g
= 5.952 mg

Volume = 200 mL = 0.200 L

Use:
Concentration of SnCl2 = mass of SnCl2 / volume
= 5.952 mg / 0.200 L
= 29.8 mg/L
Answer: 30 mg/L