# Chem question curious?

1. Jun 17, 2005

### joejo

chem question....curious?!

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

2. Jun 17, 2005

### NewScientist

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

-NewScientist

3. Jun 17, 2005

### 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...

4. Jun 17, 2005

### HallsofIvy

Staff Emeritus
For example, HCl and NaCl.

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

5. Jun 17, 2005

### joejo

anyone else that is more confident

6. Jun 17, 2005

### DrMark

Consider acetic acid:

HOAc <=> H+ + OAc-

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

7. Jun 17, 2005

### DrMark

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

8. Jun 17, 2005

### 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...

9. Jun 18, 2005

### 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

10. Jun 18, 2005

### VietDao29

If HX is a strong acid, then when add HX to water, you will have
$$HX \rightarrow H^+ + X^-$$
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
$$HX \rightleftharpoons H^+ + X^-$$
So after that, if you add NX (a salt) into water.
$$NX \rightleftharpoons N^+ + X^-$$
What can you say about the [X-] at equilibrium state? (It increases).
So what will happen to the H+?
$$\frac{[H^+] [X^-]}{[HX]} = K_C = const$$
Viet Dao,

11. Jun 18, 2005

### 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,.

12. Jun 18, 2005

### 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.

13. Jun 18, 2005

### joejo

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

14. Jun 18, 2005

### GCT

15. Jun 18, 2005

### 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....

16. Jun 18, 2005

### 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.

17. Jun 18, 2005

### joejo

no i dont...

18. Jun 18, 2005

### DDS

A buffer is simply a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. Buffers work by reacting with any added acid or base to control the pH.

Thus if the salt of the conjugate pair was added to the buffer mixture then there would not be a change in pH due to the fact that the buffers equilibrium will shift to accomadte the addition of the conjugate salt.

Is that clear?

19. Jun 18, 2005

### GCT

actually there would be a change in pH, the pH of a buffer system can vary depending on the ratio of the conjugates. Your explanation of buffers seems adequate enough, if the OP doesn't understand it then he/she needs to refer to a text to get a full explanation instead of sitting around here for an epiphany.

Your original question asks what happens when you add the salt of a weak acid to a weak acid. Explanation...buffers, it's simple as that. If you're asking for an entire tutorial on buffers, well I'll let someone else do that, but you'll get a better and more accurate answers simply by reading a text.

20. Jun 18, 2005

### DDS

Well you dont exactly know if the ph will change because your not given any concentration exactly. But if a conjugate salt is added and the the concentration is less then the buffer capacity then the pH will change. You cant just simply say the pH will change. It will only change if the concentration is over buffer capacity