Solving for a reactant in a solution

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 iron(III) chloride, which would react with silver nitrate solution like this:

FeCl3(aq) + 3AgNO3(aq) → 3AgCl(s) + FeNO33(aq)

The chemist adds 60.0m 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 5.7mg of silver chloride.Calculate the concentration of iron(III) chloride contaminant in the original groundwater sample. Be sure your answer has the correct number of significant digits.

Answer 1

FeCl3(aq) + 3AgNO3(aq) → 3AgCl(s) + FeNO33(aq)

Hi, Carsyn I'm Eddie I can help with this chemistry problem. Solubility problems involve molecules that form precipitates or solids in the solution. A solubility table or the problem will reveal what ions form solids or precipitates when dissolved together. The AgCl information can be used to solve the concentration of iron(III) chloride.

In these types of problems, it's important to write the given values. Also, take note of which substances the values belong to.

m = 5.7 mg , C= 60 M, V= 250mL

The above is ok but be sure to know the chemical these values correspond to. 5.7 mg of AgCl and 60M of AgNO₃ even though both are made of silver they are completely different chemicals. When converting from different substances you need moles so let's use the mass number.

5.7 mg of AgCl needs to be converted to grams there's 1000 mg for 1 gram so divide by 1000.

.0057 g of AgCl now we need the molar mass to go to the periodic table and find the molar mass of Ag & Cl then add them.

107.87 + 35.45 = 143.32 g/mol of AgCl

.0057g x 1mol = 3.977 x 10^-5 moles of AgCl

143.32g

Once moles are calculated use the chemical formula to convert from AgCl to moles of FeCl₃

FeCl₃(aq) + 3AgNO₃(aq) → 3AgCl(s) + Fe(NO₃)₃(aq)

3.977 x 10^-5 moles of AgCl x 1 mole of FeCl₃ = 1.33 x 10^-5 moles of FeCl₃

3 moles of AgCl

Remember, concentration is molarity it equals moles of solute divided by liters of solution. To find concentration you also need volume it was given as 250 mL but you need it in liters so divide by 1000 mL per 1 L.

250mL is .250 L of solution

1.33 x 10^-5 moles of FeCl₃ = 5.30 x 10^-5 M of FeCl₃

.250 L of solution

Hopefully, this was very helpful Carsyn if you need more help with these problems or chemistry kindly reach out have a nice day :)