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 copper(II) chloride, which would react with silver nitrate solution like this:

CuCl 2 (aq) + 2 AgNO 3 (aq) → 2 AgCl (s) + Cu NO 3 2 (aq)

The chemist adds 62.0 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 2.8 mg of silver chloride.

Calculate the concentration of copper(II) chloride contaminant in the original groundwater sample. Be sure your answer has the correct number of significant digits. (mg/L)

Answer 1

We know that ,

Molar mass of AgCl = 143g/mol

Molar mass of CuCl2 = 134g/mol

1000mL = 1L

Given data,

Volume of ground water = 200mL = 0.200L

Mass of AgCl = 2.8mg

From the stoichiometry of chemical reaction.

2mol AgCl is formed from 1mol CuCl2 .

Or

2×143g AgCl (i,e mass of 2mol AgCl) is formed from 134g CuCl2 (i.e mass of 1 mol CuCl2)

Or

286g AgCl is formed from 134g CuCl2.

Or

286mg AgCl is formed from 134mg CuCl2

So,

1mg AgCl is Formed from (134/286)mg CuCl2.

Hence

2.8mg AgCl is Formed from (134/286)×2.8mg CuCl2.

Mass of CuCl2 = (134/286)×2.8mg

Mass of cuCl2 = 1.312mg

We also know that,

Concentration = mass/volume

Concentration of CuCl2 = (1.312mg/0.200L)

Concentration of cuCl2 = 6.559mg/L

Note:- Answer must have two significant figures because the given data (i.e mass of AgCl = 2.8mg) is two significant figures.

So,

Concentration of CuCl2 = 6.6mg/L