Acid base question!

[virtualmaster999]

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Hey everyone!

Could someone help me out with this bootcamp gen chem question:

Adding sodium oxalate to a solution of oxalic acid causes the pH to:

• A. increase due to the common ion effect.
• B. decrease due to the common ion effect.
• C. remain constant because the resulting solution is a buffer.
• D. increase due to shifting the Ka.
• E. decrease due to shifting the Ka.

So I know it cant be D/E because only Temp. can affect the K value. I picked C, but now looking at it, it's true that a buffer would form (which resists change), but it would still slightly change right? So between A/B i'm not sure why it's A.

Wouldnt the dissociation be HA-> H + A-? So adding A- (base would shift left, dec the pH)?

Anything helps.

Thanks in advance, much appreciated!

 

[cyu1526]

I believe it is because HA <-> H + A- ; so because adding [A-] shifts equilibrium to the left b/c of Le Chatelier's, the concentration of H would, in turn, decrease.
When there is less [H], the pH increases. So the answer would be choice A.
More [H] = lower pH.
pH = -log[H]

 

[510586]

Adding A- would cause a shift to the left, so there would be less H+ in the solution. Then the pH would increase (more basic) right?

 

[virtualmaster999]

Adding A- would cause a shift to the left, so there would be less H+ in the solution. Then the pH would increase (more basic) right?

Thank you both! So if I'm getting this right...when there is a shift for an acid-base equilibrium problem, you want to look at what happens to H+/OH- when the shift occurs, not HA/A?

Like this: A- + H2O -> HA+OH
Say you have sodium acetate (A-) in water. And then you add Acetic acid. You would cause a shift left, decreasing OH, increasing H+, thus dec pH?

Do I got this down?

 

[510586]

Thank you both! So if I'm getting this right...when there is a shift for an acid-base equilibrium problem, you want to look at what happens to H+/OH- when the shift occurs, not HA/A?

Like this: A- + H2O -> HA+OH
Say you have sodium acetate (A-) in water. And then you add Acetic acid. You would cause a shift left, decreasing OH, increasing H+, thus dec pH?

Do I got this down?

I think if you add liquids it doesn't affect the le chatlier shifting. Right? only aq or (g)

 

[virtualmaster999]

I think if you add liquids it doesn't affect the le chatlier shifting. Right? only aq or (g)

Yup water doesnt affect it, but it can still be in the rxn. And then the shift would be due to whatever you add then.

 

[510586]

Yup water doesnt affect it, but it can still be in the rxn. And then the shift would be due to whatever you add then.

No I'm saying if you add HA (acetic acid) isn't that a liquid?

 

[virtualmaster999]

No I'm saying if you add HA (acetic acid) isn't that a liquid?

Ahh I see what you mean. I was gonna say everything in my example is aqueous except the water. Just made the problem up on the fly, so Im not sure of the phases to be honest. I dont think it would be liquid though.