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 cadmium chloride, which would react with silver nitrate solution like this: CdCl2 (aq) + 2AgNO3 (aq) → 2AgCl (s) + CdNO32 (aq) The chemist adds 10.0m M silver nitrate solution to the sample until silver chloride stops forming. He then washes, dries, and weighs the precipitate. He finds he has collected 7.6mg of silver chloride. Calculate the concentration of cadmium chloride contaminant in the original groundwater sample. Round your answer to 2 significant digits.

Answer 1

Given reaction is : CdCl2 (aq) + 2AgNO3 (aq) → 2AgCl (s) + CdNO32 (aq)

Molar mass (g/mol) 183.3 143.3

From the above balanced reaction ,

2 mole = 2x143.3 g of AgCl produced from 1 mole = 183.3 g of CdCl2

7.6 mg = 7.6x10^-3 g of AgCl is produced from M g of CdCl2

M = ( 7.6x10^-3 x183.3)/ (2x143.3)

= 4.86x10^-3 g

Number of moles , n' = mass/molar mass

= 2.65x10^-5 moles

Concentration of CdCl2 = number of moles / volume in L

=  2.65x10^-5 moles / 0.200 L

= 1.32x10^-4 M

= 0.13 mM