Suppose an EPA chemist tests a 200.mL sample of groundwater known to be contamin

Question

Suppose an EPA chemist tests a 200.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 46.0mM silver nitrate solution to the sample until silver chloride stops forming. She then washes, dries, and weighs the precipitate. She finds she has collected 2.5mg of silver chloride.

Calculate the concentration of iron(III) chloride contaminant in the original groundwater sample.

Explanation / Answer

FeCl3(aq) +3AgNO3(aq)3AgCl(s)+FeNO3(aq)

So here

one mole of Fe+3 ions will react with three moles of AgNO3 to give Three moles of AgCl

The mass of AgCl obtained = 2.5mg

Moles of AgCl = Mass / molecular mass = 2.5 X 10^-3 / 143.5 = 0.0174 millimoles

So moles of FeCl3 present = 1/3 X moles of AgCl formed = 0.0174 / 3 millimoles = 0.0058 millimoles of FeCl3

Volume of sample = 200mL

So concentration of FeCl3 = Millimoles of FeCl3 / volume in mL = 0.0058 / 100 = 5.8 X 10^-4 M

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