SAT Chem: Molarity to be remembered?

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1

I was just wondering whether or not I need to remember the molarity of certain substances like H3O+ and H2O, etc.

2

Doesn’t molarity change based on the concentration of the solution? So 1 M H3O+ solutions and 0.1 M H3O+ solutions are both possible. I’m not sure what you’re asking.

3

Sorry, I meant density.