This is the problem:
How many millilitres of 0.421 M HCl must be added to 50.0 mL of a 0.055 M
solution of disodium malonate (Na2A) in order to prepare a buffer that has pH of a.) 6,00 b.) 3,2
Some facts, that are mentioned in the question:
- malonic acid - (H2A) - HO2CCH2CO2H
- pk1= 2,847
- pk2= 5,696
So, i had no problem with dealing with the example a.). (You have a buffer that contains [A2-] and [HA-]. As much HCl you add, as much [HA-] you get, and as much less [A2-] you have at the end. ). However, I cannot solve the b.), even though I know it is pretty much similar to a.). The correct answer for a.) 2,17 ml and for b.) 8,47 ml.
Because the buffer has pH of 3,2 it is logical, that you have mix of [HA-] and [H2A]. But I really don't know how to get these two concentrations. Is there also [A2-] in the mix?
Thanks for your help.