What is the pH of [H3O+]=1.0 M? How do I solve for it?
For example, a solution in which
Protons have a natural tendency to combine with water molecules to form hydronium ions in aqueous solutions. (In fact, hydrogen ions won't even exist in the first place due to the overwhelming quantity of water.) 
Soult, Allison. “8.6: The PH Concept.” Chemistry LibreTexts, Libretexts, 12 Oct. 2017, chem.libretexts.org/LibreTexts/University_of_Kentucky/UK%3A_CHE_103_-_Chemistry_for_Allied_Health_(Soult)/Chapters/Chapter_8%3A_Properties_of_Solutions/8.6%3A_The_pH_Concept
 Kahlon (UCD), Avneet, and Stephen Lower, Professor Emeritus (Simon Fraser U.). “The Hydronium Ion.” Chemistry LibreTexts, Libretexts, 8 Oct. 2017, chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Acids_and_Bases/Acids_and_Bases_in_Aqueous_Solutions/The_Hydronium_Ion