* RbOH: Rubidium hydroxide. Since rubidium (Rb) only forms one cation (+1), we don't need to specify the charge with Roman numerals.
* Ca2C: This is a bit tricky because it's not a typical ionic compound. It would be better represented as CaC2, which is calcium carbide. Calcium (Ca) is +2, and carbide (C2) is -2, so we don't need Roman numerals.
* K2SO4: Potassium sulfate. Potassium (K) only forms a +1 ion, and sulfate (SO4) is -2, so we don't need Roman numerals.
* Al2S3: Aluminum sulfide. Aluminum (Al) is +3, and sulfide (S) is -2, so we don't need Roman numerals.
Key Point: The Stock system is mainly used when a metal can form multiple cations. For example, iron (Fe) can be +2 (ferrous) or +3 (ferric). We would use Roman numerals to indicate the charge in these cases.
