A buffer consists of 0.16 M KHCO3 and 0.39 M K2CO3. Carbonic acid is a diprotic acid with Ka1 = 4.5 10-7 and Ka2 = 4.7 10-11.
(a) Which Ka value is more important to this buffer?
Ka1
Ka2
I don't understand this one at all. How do I determine which Ka value is more important to the buffer?
A very good question that vexed me for years. Here is how I handle it.
H2CO3 ==> H^+ + HCO3^-
HCO3^- ==> H^+ + CO3^2-
k1 = (H^+)(HCO3^-)/(H2CO3)
k2 = (H^+)(CO3^2-)(HCO3^-)
and Henderson-Hasselbalch equation you will be using is
pH = pKa(1 or 2) + log(base)/(acid)
Look at the problem. The acid in the problem is HCO3^- and the base is CO3^2-
Does k1 have HCO3^- and CO3^2- in it? No. Does k2? Yes, it has CO3^2- in the numerator and HCO3^- in the denominator. Therefore, you use k2. That always makes it easy to spot. It is even more valuable when dealing with phosphate buffers because you have k1, k2, and k3 and you must choose which to use. This system always works.
Thank you so much!
Thank you for a very good explanation! :)
Thanks alot
But is it considered as acidic buffer or basic buffer solution?
Well, determining which Ka value is more important to the buffer is like trying to decide which ingredient is more important in a recipe - a real pickle! But don't worry, I'm here to bring some clarity to this bubbly situation.
In this case, we have a diprotic acid (carbonic acid), which means it can donate two protons. Ka1 and Ka2 represent the acid dissociation constants for the first and second proton, respectively.
To determine which Ka value is more important, we look at the relative magnitudes. In this case, Ka1 is 4.5 x 10^-7, while Ka2 is a whopping 4.7 x 10^-11.
Since Ka1 is significantly larger than Ka2, it means that the first proton dissociation (Ka1) is more significant in the buffer. In other words, the first proton tends to dissociate more readily compared to the second proton.
So, the answer to this riddle is Ka1! It's the MVP (Most Valuable Proton) in this buffer.
To determine which Ka value is more important to the buffer, you need to consider the relative concentrations of the acid and its conjugate base in the buffer solution. In this case, the buffer is made of a mixture of KHCO3 (which is the acid component) and K2CO3 (which is the conjugate base component).
Ka1 represents the equilibrium constant for the reaction of carbonic acid (H2CO3) with water to form bicarbonate ions (HCO3-) and hydronium ions (H3O+): H2CO3 + H2O ⇌ HCO3- + H3O+.
Ka2 represents the equilibrium constant for the ionization of bicarbonate ions (HCO3-) into carbonate ions (CO32-) and hydronium ions (H3O+): HCO3- + H2O ⇌ CO32- + H3O+.
In a buffer system, the acid-base pair should be present in roughly equal concentrations to effectively resist changes in pH.
In this case, since the buffer consists of a mixture of KHCO3 and K2CO3, there will be a significant amount of HCO3- ions present, which are formed from the dissociation of carbonic acid (H2CO3) in water. Therefore, the concentration of HCO3- ions will be relatively high compared to the concentration of H2CO3.
Since Ka1 represents the equilibrium constant for the reaction involving H2CO3 and HCO3- ions, it will be more important to the buffer system.
In conclusion, Ka1 is more important to this buffer because the concentration of HCO3- ions (which is involved in the reaction represented by Ka1) is higher compared to the concentration of H2CO3.