Nickel(II) sulfide, NiS, occurs naturally as the relatively rare mineral millerite. One of its occurrences is in meteorites. To analyze a mineral sample for the quantity of NiS, the sample is dissolved in nitric acid to form a solution of Ni(NO₃)₂.

The aqueous solution of Ni(NO₃)₂ is then reacted with the organic compound dimethylglyoxime (C₄H₈N₂O₂) to give the red solid Ni(C₄H₇N₂O₂)₂. 

Suppose a 0.468-g sample containing millerite produces 0.206 g of red, solid Ni(C₄H₇N₂O₂)₂. What is the mass percent of NiS in the sample? 

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NiS(s) + 4HNO3(aq) – Ni(NO3)2(aq) + 2 NO₂(g) + 2 H₂O(l) + S(s)