Chemistry - Volumetric determination

[h6872]

Hi... I x-posted this to the other chemistry board..but that might've been the wrong place...

I recently performed an experiment to determine the % Cl (by mass) of an unknown chloride sample containing both potassium and sodium. To determine this, we did a Fajans method titration using 0.1 M AgNO3.
A 1.0233g sample of the unknown chloride sample was diluted and titrated with the silver nitrate. Approximately 15.70 mL of the silver nitrate was used.
Okay, so, I understand that to determine the % Cl by mass, I should determine a final mass and divide it by the original mass of the entire chloride sample. This may seem like a stupid question, but what final mass are we determining? From the total volume of silver nitrate used, I can figure out the moles of silver nitrate used. Where do I go from here?
Please help me!
Thanks,
Heather

 

[mrjeffy321]

You say you used "Approximately 15.70 mL of the silver nitrate"...but what was the concentration of the AgNO3 ? This will determine the number of moles used.

The net ionic equation of the titration you preformed was,
Cl- (aq) + Ag+ (aq) --> AgCl (s)

1 mole of Chloride ions react with 1 mole of Silver +1 ions to form 1 mole of Silver Chloride precipitate.

By knowing the number of moles of AgCl formed or the number of Ag+ ions used to fully react with the Chloride ions in solution, one can determine the number of moles of Cl- originally present.
Apparently, you know the number of moles of AgNO3 used (by way of the volume used * concentration). Since 1 mole of AgNO3 disassociates to form 1 mole of Ag+ ions, if you know the # of moles of AgNO3, you know the # of moles of Ag+ (1:1 ratio), and thus you know the # of moles of Cl- ions originally present in solution.

If you know the number of moles of Cl-, you can calculate the mass of the Cl- since the Cl- ion has the same molar mass as Chlorine (the extra electron is negligible).
Once you calculate the mass of the Cl-, you can find the % Cl- by mass in the original, mystery, substance since you know the original substance's mass.
% Cl- by mass = (mass of Cl- / mass of total substance) * 100