calculating pH

[Ryan Fang 1D]

I recall in Friday lecture, Dr.Lavelle was talking about how if the concentration of H3O+ is less than 10^-7, the solution then the solution is considered neutral. He brought up numbers like 0.0000001 M H3O+ and 0.0000000001 M H3O+ and he said that if you take the negative log of the smaller number, you would get pH = 10, but in reality the solution is neutral. I'm sorry if this information is incomplete or confusing but I was just really confused about that part of the lecture. Can someone help? Thank you!

[Emily Mei 1B]

I recall in Friday lecture, Dr.Lavelle was talking about how if the concentration of H3O+ is less than 10^-7, the solution then the solution is considered neutral. He brought up numbers like 0.0000001 M H3O+ and 0.0000000001 M H3O+ and he said that if you take the negative log of the smaller number, you would get pH = 10, but in reality the solution is neutral. I'm sorry if this information is incomplete or confusing but I was just really confused about that part of the lecture. Can someone help? Thank you!

I was confused about this too, but for some more details on what he said: If we calculated [H3O+] < 10^-7, we consider the solution neutral because we know autoprotolysis (self-ionization of water) generates 10^-7 mol/L H3O+. The difference between our calculated amount and the actual amount of [H3O+] that is already in water is really small.

[Aya Shokair- Dis 2H]

He was referring to if you add a small quantity of a weak acid and you calculate the pH/ [H3O+], you'll get what looks like a strong base.
He was saying that should make you question it because why would adding a small amount of weak acid result in a strong base?
So, he brought of the point that the acid might of released 10^-10 H3O+ but that is not the only H3O+ in the solution. Water alone produces 10^-7. So, you really have 10^-7 H3O+ ions.
This is analogous to to having 1 penny (10^-10) and a dollar (10^-7). If you say you only have 1 penny that is wrong because you're missing and entire dollar from your answer. So, it is more logical to say you have a dollar and that'll be logically consistent because the penny won't make a difference.
I hope that helps!!

[Ryan Fang 1D]

He was referring to if you add a small quantity of a weak acid and you calculate the pH/ [H3O+], you'll get what looks like a strong base.
He was saying that should make you question it because why would adding a small amount of weak acid result in a strong base?
So, he brought of the point that the acid might of released 10^-10 H3O+ but that is not the only H3O+ in the solution. Water alone produces 10^-7. So, you really have 10^-7 H3O+ ions.
This is analogous to to having 1 penny (10^-10) and a dollar (10^-7). If you say you only have 1 penny that is wrong because you're missing and entire dollar from your answer. So, it is more logical to say you have a dollar and that'll be logically consistent because the penny won't make a difference.
I hope that helps!!

Ok, thank you, this explanation makes a lot of sense. However, if an acid were to produce 10^-7 H3O+ ions where we cannot call that amount negligible and we also cannot call the autoprotolysis 10^-7 H3O+ negligible would we have to add these two concentrations together before we find the pH?