if [H3O+] is less than 10^-7

Acidity $K_{a}$
Basicity $K_{b}$
The Conjugate Seesaw $K_{a}\times K_{b}=K_{w}$

Abigail Urbina 1K
Posts: 102
Joined: Thu Jul 27, 2017 3:01 am

if [H3O+] is less than 10^-7

Can someone please re-explain/clarify why it is that the solution is still considered neutral even when [H3O+] < 10^-7 ? How exactly does this concept tie in with the reasoning that autoprotolysis generates 10^-7 mol/L ?

Curtis Tam 1J
Posts: 105
Joined: Thu Jul 13, 2017 3:00 am

Re: if [H3O+] is less than 10^-7

By definition, autoprotolysis generates 10^-7 in concentration for hydronium. When we add such a tiny amount of acid, the amount of hydronium it produces may be so minute as to say perhaps 10^-15. However, when we calculate pH, we get something greater than 7 which cannot be possible because we are working with an acid. Essentially, you ignore what the weak acid has contributed and instead look at what the water has contributed in terms of hydronium ions. It's almost like saying water gives 100 dollars. If the weak acid gives 0.00001 cents, you can essentially say the solution still has 100 dollars and is therefore neutral.