pH below or above 7 or neutral

AlexNguyen15
Posts: 66
Joined: Fri Sep 25, 2015 3:00 am

pH below or above 7 or neutral

How to tell if the pH of a salt is equal to, greater than, or less than 7?
For example with Na2CO3 and Cu(NO3)2.
How can you tell by looking at the formula if something is an acid, base, or neutral?

Rachna Goli 3E
Posts: 24
Joined: Fri Sep 25, 2015 3:00 am

Re: pH below or above 7 or neutral

To calculate the pH of a solution, first calculate the H+ or H3O+ ion concentration in the solution. The pH = -log([H+]) and based on the number you get (<7, 7, or >7), you can infer whether the solution is acidic, neutral, or basic. You can use the general rule that acids usually produce H+ or H3O+ ions, and bases usually produce OH- ions in solutions to categorize which ones are acids and bases.

Samantha Rundle 3K
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Re: pH below or above 7 or neutral

If you calculate the pH and it comes out neutral, is that similar to being a very weak acid or is it a different state completely? Also when a weak acid has a pH less than 10^-7, does that mean that it barely dissociates, because strong acids and bases dissociate completely?

AlexNguyen15
Posts: 66
Joined: Fri Sep 25, 2015 3:00 am

Re: pH below or above 7 or neutral

But without any calculations, how can you tell by looking at the formula only if it is a base, acid, or neutral?

Aleck Sun 1J
Posts: 46
Joined: Fri Sep 25, 2015 3:00 am

Re: pH below or above 7 or neutral

In the course reader, it mentions that small highly charged metal cations act as lewis acids to increase [H3O+]. This would include Fe3+, Cr3+, Al3+, Fe2+, Cu2+, Ni2+ (all aq). So, I believe that means that any salt with these metals will be acidic. On the contrary, salts that contain the conjugate base (anion) of a weak acid will raise the pH by picking up protons. This includes CH3CO2-, CO3 2-, PO4 3-, OH-, F-, O 2-, and S 2-.
Based on this information, I would say that Na2CO3 is basic and Cu(NO3)2 is acidic.
Na+ in general will not affect pH.

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