[govibe]

I'm having a bit of a problem with this...

1. "Douglasite is a mineral with the formula 2 KCl·FeCl2·2 H2O. Calculate the mass percent of douglasite in a 348.0 mg sample if it took 37.20 mL of a 0.1000 M AgNO3 solution to precipitate all of the Cl- as AgCl. Assume the douglasite is the only source of chloride ion."
Since this is a weird molecule, what would be the molecular equation of douglasite reacting with AgNO3? Does the coefficient of 2 distribute over the FeCl2 in addition to the KCl? I am able to find moles of AgNO3 but that is it...

[Borek]

In the douglasite crystal there are two K⁺ per each Fe²⁺ - you may write it as (KCl)₂FeCl₂(H₂O)₂ (well, don't do that, but that's how it describes substance composition). Otherwise you just treat it as if it was one molecule.

[govibe]

so how do you figure out the equation when it reacts with the AgNO3?

[AWK]

Write down a formula of douglasite, eq  as: K₂FeCl₃H₄O₂
and balace this reaction:
K₂FeCl₄H₄O₂ + AgNO₃ = AgCl(s) + KNO₃ + Fe(NO₃)₂ + H₂O

[govibe]

^How did you know to write the equation like that? (with the specific molecules)

[Borek]

Observe anions and cations presnt - once you remove Ag⁺ and Cl^- there are K⁺, Fe³⁺ and NO₃^- left - not much choice when it comes to decide what salts are present in the solution.

This question should be done with net ionic reaction, which you likely don't know yet.