A buffer consisting of H2PO4- and HPO42-, helps control the pH of physiological fluids. Many carbonated soft drinks also use this buffer system. You were asked to prepare this buffer from K2HPO4 and KH2PO4. Identify the week acid and base components of this buffer solution and justify your choice.
K2HPO4 and KH2PO4
The acid is the one with more H atoms; the base is the one with the fewer H atoms. So KH2PO4 is the acid and K2HPO4 is the base. The equilibrium is
H2PO4^- + OH^- ==> HPO42- when strong base is added and
HPO4^2- + H^+ ==> H2PO4^- when strong acid is added.
To identify the weak acid and base components of the buffer solution made from K2HPO4 and KH2PO4, we need to understand how these compounds dissociate in water.
K2HPO4: This compound dissociates into K+ ions and HPO42- ions in water.
KH2PO4: This compound dissociates into K+ ions and H2PO4- ions in water.
In the buffer solution, H2PO4- acts as the weak acid, and HPO42- acts as the weak base.
The justification for this choice lies in their ability to act as both an acid and a base. H2PO4- can donate a proton (H+) to water, making it a weak acid, while HPO42- can accept a proton (H+) from water, making it a weak base.
The pH of a solution is determined by the balance between hydrogen ions (H+) and hydroxide ions (OH-) in the solution. The weak acid (H2PO4-) and weak base (HPO42-) in the buffer can react with any added hydrogen ions or hydroxide ions, helping to keep the pH relatively stable.
Overall, the presence of H2PO4- and HPO42- in the buffer system allows it to resist significant changes in pH when acids or bases are added.
In the buffer solution consisting of H2PO4- and HPO42-, the weak acid component is H2PO4- (dihydrogen phosphate ion) and the weak base component is HPO42- (hydrogen phosphate ion).
Here's the justification for this choice:
H2PO4- can act as a weak acid because it can donate a proton (H+ ion) to the solution. It can undergo partial dissociation in water to form HPO42- and release H+ ions:
H2PO4- ⇌ H+ + HPO42-
Thus, in the presence of excess H2PO4-, it acts as a proton donor and helps in maintaining the pH of the solution.
Similarly, HPO42- can act as a weak base because it can accept a proton (H+ ion) from the solution. It can undergo partial association in water to form H2PO4- by accepting H+ ions:
HPO42- + H+ ⇌ H2PO4-
In the presence of excess HPO42-, it acts as a proton acceptor and helps in maintaining the pH of the solution.
Therefore, the H2PO4- and HPO42- ions are the weak acid and weak base components of this buffer system, respectively.