Why is the reaction quotient Q always equal to one at standard state?
As already said in the comments, you cannot use any unit you want in the reaction quotient; it has to be formulated in terms of activity (for solutes) or fugacity (for gases).
The &quot;standard state&quot; of a chemical species is a reference state in which the properties of the chemical species are defined. This standard state is a theoretical state, it does not need to be practically reachable by the chemical species. By definition, solutes in standard state have an activity of 1, and gas have a fugacity of 1. Therefore, whatever you reaction, if all the species are assumed to be in standard state, the reaction quotient will be a product of 1s, that is, 1.