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 19.0mM silver nitrate solution to the sample until silver chloride stops forming. She then washes, dries, and weighs the precipitate. She finds she has collected 2.9mg of silver chloride.

Calculate the concentration of tin(II) chloride contaminant in the original groundwater sample. Round your answer to 2 significant digits.

Answer 1

SnCl_2(aq) + 2AgNO_3(aq) -------> 2AgCl_(s) + Sn(NO₃)_2(aq)

We have 250 mL of sample, and we add 19 mM of AgNO₃ (0.019 M)

the mass obtained was 2.9 mg of AgCl (0.0029 g)

With this, we can calculate the number of moles (which should be the same number of moles of silver nitrate).

moles of AgCl = 0.0029 g / (35.45 + 106.87) = 2.037x10^-5 moles

Now, with this moles, we can calculate the moles of SnCl₂

moles of SnCl₂ = 2.037x10^-5 / 2 = 1.0187x10^-5 moles

Finally, the concentration:

C = 1.0187x10^-5 / 0.250 = 4.0748x10^-5 M

Hope this helps.