chem question....curious?!

joejo

hi guys i was just wondering that if a solution of a weak acid has a salt of the same acid added to it, what would happen to the pH??

can anyone help me out....thanks in advance

NewScientist

A salt of the same acid? What do you mean?!

-NewScientist

joejo

well i dont know...it was on my exam that i just had today..and i didnt know how to answer it...thats why im soo curious...

can anyone please help me out..

HallsofIvy

For example, HCl and NaCl.

I don't believe that a salt will change the pH.

joejo

anyone else that is more confident

DrMark

Consider acetic acid:

HOAc <=> H⁺ + OAc^-

What would happen to this equilibrium upon increasing the concentration of OAc^- say with NaOAc ?

DrMark

And btw welcome to the wonderful world of buffer pairs (which I'm preety sure where this is going) ;)

joejo

im so lost....i dont know wat the answer was...now im scared to see my mark...can someone plz explain this to me...

NewScientist

So - let me get this clear we are talking about a salt that is comprised of :

Metal+(-ve ion from acid) being added to a solution of Hydrgoen+(-ve ion)

At this point, one wuold typically think of displacement, however, the two -ve ions are the same and so i wonder if instead of displacement do we not see a HCl precipitate and NaCl becoming the solution? Just a thought.

-NewScientist

VietDao29

If HX is a strong acid, then when add HX to water, you will have
[tex]HX \rightarrow H^+ + X^-[/tex]
So after that, if you add NX (a salt) into water. Nothing will happen to the pH. Because all the HX have become H+ And X- (HX is a strong acid). HCl is an example.
---
But if HX is a weak acid, then when add HX to water, you will have
[tex]HX \rightleftharpoons H^+ + X^-[/tex]
So after that, if you add NX (a salt) into water.
[tex]NX \rightleftharpoons N^+ + X^-[/tex]
What can you say about the [X-] at equilibrium state? (It increases).
So what will happen to the H+?
[tex]\frac{[H^+] [X^-]}{[HX]} = K_C = const[/tex]
Viet Dao,

primarygun

it depends whether the salt is acid salt or normal salt.
Acid besides monobasic acid usually form more than one kind of salts.
That includes hydrogen-containing salt, namely, acid salts.
They dissolve in water to give out hydrogen ions,.

GCT

Dr. Mark has hit the spot, upon adding a certain concentration of the salt, common ion effect will take place and neither of the weak acid or its conjugate salt will contribute to the pH, that is neither the acid will dissociate significantly or the conjugate base act to hydrolyze the water. Thus you will effectively have a buffer solution, the small variations in pH will be due to the autodissociation of water in relation to the equilibrium constant.

joejo

sorry there are soo many answers and you guys are losing me...can someone summarize the correct answer please...thanks again

GCT

The answer is simple...read up on buffers in your chemistry text.

joejo

I don't have my textbook anymore....thats why im asking up here..I finished school and handed in my textbook..this was a question of my grade 12 chemistry exam...i'm just curious...could someone please help me out....

GCT

Don't you know what a buffer is? Well, again, you would have a buffer solution (upon adding a certain amount of the salt). The pH can be calculated simply by plugging into the henderson-hasselbach equation.

joejo

no i dont...