MathType works within Pages in the way Viking has described, and has a template that will apply both a subscript and a superscript to the same base -- something like this:

You'll also want to set your style definitions in MathType (Style menu > Define) to non-italic variables. This option is set to italic by default, since in mathematics, variables are typically italicized. If you have other questions about using MathType for chemical equations (physical chemistry only -- it doesn't do benzene rings or other structures), feel free to ask here, which is OK if it involves Apple software too, but it makes more sense to write us directly at support at dessci dot com, if the question doesn't have anything to do with Apple products.
Bob Mathews
Design Science