Douglasite is a mineral with the formula 2KCl • FeCl2 • 2H2O. Calculate the mass percent of douglasite in a 455.0-mg sample if it took 37.20 mL of a 0.1000 M AgNO3 solution to precipitate all the Cl2 as AgCl. Assume the douglasite is the only source of chloride ion.

The mass percentage is 63.3%

Explanation:

First we need to know how much of the AgNO3 we are using to precipitate the chloride, for this we use:

(37.2 ml * 0.1 M) / 1000 ml = 0.00372 moles of AgNO3 which means 0.00372 moles of silver.

the relation between Ag and Cl in the precipitation is 1:1, so we can determine that in our sample we have 0.00372 moles of Cl - and in grams we have 131 mg.

Now we need to know how much of the Douglasite is actually chloride. For that we calculate the molar mass:

Mr = 2*39 + 4*35.45 + 55.85 + 2*18 = 311.65 g/mole

Of that 311.65 grams, 141.8 are Cl. So we could calculate how much of the sample is actual Douglasite:

141.8 g Cl - -> 311.65 g Douglasite

0.131 g Cl - -> x = 0.288 g douglasite.

The sample is 0.455 and represents 100 %

Then, the 288 mg of douglasite is the 63.3 %