Question

What is the pH of [H3O+]=1.0 M? How do I solve for it?

Answer 1

`"pH"=0.00`

Explanation:

By definition, `"pH"=-log["H"^+]`
where `["H"^+]` the proton concentration in the solution in `"mol"*"dm"^(-3)`.

For example, a solution in which `conc.("H"^+)=1.0color(white)(l)"mol"*"dm"^(-3)` has `"pH"=-log(1.0)=0.00`. Note that only numbers on the right of the decimal place of logarithms count towards significant figures. [1]

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.) [2]

`"H"^(+)(aq)+"H"_2"O"(l)->"H"_3"O"^(+)(aq)`

Thus `"pH"=-log["H"^(+)]=-log["H"_3"O"^(+)]=0.00`

References
[1]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

[2] 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