[sarah]

PLease help me answer this question i dont even understand how to start this question and its from my assignment which is due very soon. I will really appreciate the help. Thanks

Solid black CuS is exceedingly insoluble. A chemist observed that it would not
readily dissolve in HCl, but would react with concentrated HNO3 resulting in
solution containing Cu2+, among other things. The chemist also observed the
formation of a red-brown gas as the reaction proceeded. The chemist in question
was a keen observer, but careless. When performing the reaction, he forgot to
write down the mass of the CuS that he used. Being a conscientious chemist in
all aspects of your work, you have offered to solve the problem for the careless
chemist. The solution prepared by the chemist was transferred quantitatively
into a 250 mL volumetric flask and diluted to the mark after which a 20.0 mL
aliquot was treated with excess KI. The resulting solution required 77.33 mL of
0.00812 M Na2S2O3 solution to reach equivalence. What mass of CuS did the
sloppy chemist use?

[Sam (NG)]

Start by identifying the brown gas, and therefore what type of reaction has occurred.

[Borek]

More precisely: write down equations of ALL reactions that took place.

To be precise, CuS dissolution is of the least importance here, as long as you are aware of the fact that amount of copper and amount of copper sulfide is - in terms of moles - identical.