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Question: I was confused about the last statement you made at the end of class: "If [H3O+] < 10-7, then the solution is considered neutral because we know that as a result of autoprotolysis they are 10-7 M H3O+." I am have trouble understanding how a solution would be neutral if [H3O+] is less than 10-7.
Answer: Neutral water has 10-7 M H3O+. If one calculates that a compound when added to neutral water produces H3O+ (equivalent to a concentration of 10-10 M H3O+) then the compound is hardly increasing the H3O+concentration above 10-7 M. In other words, add 10-10 M H3O+ + 10-7 M H3O+ and then take the log of the sum and the pH is 6.999 hardly different from pH 7, which is the pH of the water before the compund was added.
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