Lecture note from 1/20/23 chem 14B

[chantalaguilar3C]

On the last slide the professor wrote a note talking about concentration saying "If [H3O+] < 10^-7 then solution is considered neutral because we know autoprotolysis generates 10^-7 mol/L H3O+", Im still confused on what type of problems this note would be applicable or what it really means.

[Andre Contreras 1I]

This is saying that in reactions where the [H3O+] concentration is less than that of what is generated in autoprotolysis then we say that it is is neutral. For example, if you got a concentration of 10^-9 which is less than 10^-7 then you would calculate a pH of 9, however this reaction would not create a basic solution as it is so minute in weak acid and base reaction that it would not change the neutral pH.

[Vincent Tran 3B]

Water has 10^-7 M of H3O+ so when a compound is producing an H3O+ concentration of less than 10^-7, the difference in H30+ is so small that it doesn't affect the pH of the solution by much. Say an arbitrary concentration of 10^-8 M of H3O+ was produced by a compound. Because its concentration is so small, it will not change the pH of the solution by much. For example, if a compound is added to water and produces 10^-8 M of H3O+, when added to the concentration of H3O+ concentration of water(10^-7), the -log of those concentrations added will still be close to 7. This is true because of the H3O+ generated by autoprotolysis