DATBootcamp Chemistry Question

[Tooth Worthy]

"Adding sodium oxalate to a solution of oxalic acid causes the pH to increase due to the common ion effect."

I understand why adding base would increase the pH but why would it be due to the common ion effect I have no idea...Any thoughts on this?

 

[Ari Rezaei]

Whenever you're using a weak acid, like oxalic acid, write the equation as:

HA <--> H+ + A-

HA is oxalic acid, the H+ is what makes something acidic or basic. The A- is the oxalate.

By adding oxalate, a common ion, you push the equation to the left. In the process, you decrease the amount of H+, making the solution more basic.

 

[Tooth Worthy]

Makes sense. Thank You!

 

[bluemoon27]

I dont understand how it becomes more basic because doesnt it go back to being HA which is an acid? I understand why it goes to left and that H+ is decreased, but its becoming an acid HA, so thats why I dont get how the solution gets basic. Please can you clarify. Thanks so much!

Whenever you're using a weak acid, like oxalic acid, write the equation as:

HA <--> H+ + A-

HA is oxalic acid, the H+ is what makes something acidic or basic. The A- is the oxalate.

By adding oxalate, a common ion, you push the equation to the left. In the process, you decrease the amount of H+, making the solution more basic.

 

[Ari Rezaei]

Sure, pH is a logarithmic measure of hydrogen ion concentration, NOT the concentration of the actual acid. What makes something acidic is how much it dissociates.

Oxalic acid is a weak acid because it stays mostly in the HA form, and only a little of it dissociates into H+ and A-. The H+ is what we use to measure pH.