[gcatal]

For my lab, we were given the following equation:

Pb(NO3)2 (aq)  + 2KI (aq) →  PbI2 (s)  + 2KNO3 

( 25mL of 0.05 M lead nitrate solution was mixed with 1.4mL of 0.25M potassium iodide solution)

We were asked to identify the limiting agent before doing the lab.

My calculations indicated that .00125 moles of Pb & .00035 of I were present before the reaction & .001075 moles of Pb & 0 moles of I would be present after the reaction, with I being the limiting agent.

So I did the experiment, filtered out the precipitate, and then conducted a test for excess lead ion (by adding potassium phosphate) & excess iodide ion (by adding silver nitrate) on the filtrate. They both came up positive in repeated tests and I don't understand why. Shouldn't the I have been used up? And only lead should have been present since it was in excess? Can anyone give me a sense of what went wrong here?

[AWK]

Nothing wrong!
PbI₂ is sufficiently soluble in water to give a positive test with Ag⁺.

[gcatal]

Okay - I guess I'm just not clear on why the limiting agent wasn't used up. Is it that this reaction doesn't go to completion so both reactants are still present after the reaction occurs?

For another reaction, Pb(NO3)2  (aq) +  Na2CO3 (aq)  →   PbCO3 (aq)  +   2 Na(NO3) (aq), my test on the filtrate for the limiting agent (CO3) came up negative. Only Pb was present after the reaction took place. I don't understand why the limiting agent got used up in this reaction but not in the other.

 Is it that this reaction doesn't go to completion so both reactants are still present after the reaction occurs?

[AWK]

If you cannot find CO₃^2- - it means  carbonate was a limiting reagent

[gcatal]

That's what I figured - my question is, if iodide is the limiting reagent in this reaction (Pb(NO3)2 (aq)  + 2KI (aq) →  PbI2 (s)  + 2KNO3  (aq)), why am I finding excess iodide ion in my filtrate?

It should be absent, like the CO3 is in the first reaction I did (Pb(NO3)2  (aq) +  Na2CO3 (aq)  →   PbCO3 (aq)  +   2 Na(NO3).

 Is there any reason why the limiting reagent would not be used up and still be present in the products of a reaction?

[Borek]

That's what I figured - my question is, if iodide is the limiting reagent in this reaction (Pb(NO3)2 (aq)  + 2KI (aq) →  PbI2 (s)  + 2KNO3  (aq)), why am I finding excess iodide ion in my filtrate?

As AWK wrote - PbI₂ solubility is high enough for the I^- to precipitate Ag⁺. Iodide is a limiting reagent, but it is still present in the saturated solution of PbI₂.

Do you know what solubility products are? Check what is solubility product fro PbI₂ and for AgI.