Regents Chemistry Exam August 2022
35 What is a chemical name of the compound CuS? (1) copper(I) sulfide
(2) copper(I) sulfate
(3) copper(II) sulfide
(4) copper(II) sulfate
Explanation: CuS is an ionic compound because it has a metal with a nonmetal. To name an ionic compound with a metal and a nonmetal, we say the name of the metal followed by nonmetal ending in "ide". This eliminates choices 2 and 4 since they end in "ate" (that would be polyatomic ion).
Copper is also a transition metal and we need to use Roman numerals when there is a transition metal. For ionic compounds, we exchange their oxidation numbers (charges) to arrive at the formula. Sulfur is group 16 on the Periodic Table with an oxidation number of -2 (top right number). The formula is CuS which means one to one ratio of the atoms. If sulfur is -2 then copper has to be +2 to balance sulfur out and make it 1 to 1 ratio.