Write the chemical equation showing dihydrogen phosphate and hydrogen phosphate conjugate acid-base relationship. Identify the acid and base. Explain how the equilibrium is shifted as buffer reacts with an acid and reacts with a base.

H2PO4^- + OH^- ==> HPO4^2- + H2O

The acid is the one with more H and the base the one with fewer H atoms. The above reaction shows how the buffer works with base to keep the pH the same.

The chemical equation showing the dihydrogen phosphate and hydrogen phosphate conjugate acid-base relationship is as follows:

H₂PO₄⁻ + H₂O ⇌ H₃O⁺ + HPO₄²⁻

In this equation, H₂PO₄⁻ is the dihydrogen phosphate, which is acting as a base and accepts a proton (H⁺) from water (H₂O) to form H₃O⁺, which is the hydronium ion. Simultaneously, dihydrogen phosphate loses a proton to form HPO₄²⁻, which is the hydrogen phosphate ion.

In this equilibrium, H₂PO₄⁻ is the acid as it donates a proton (H⁺), and HPO₄²⁻ is the conjugate base as it accepts the proton. Similarly, HPO₄²⁻ can act as a weak acid and undergoes the same reaction to form PO₄³⁻, the phosphate ion, and H₂PO₄⁻ (dihydrogen phosphate) acts as the conjugate base.

Now, let's discuss how the equilibrium is shifted when the buffer reacts with an acid or a base. A buffer is a solution that resists changes in its pH when an acid or base is added. It contains a weak acid and its conjugate base or a weak base and its conjugate acid.

When an acid is added to the buffer system, it donates protons to the buffer's weak base. This results in a decrease in the concentration of the weak base and an increase in the concentration of its conjugate acid. As a result, the equilibrium shifts to the left, favoring the production of more dihydrogen phosphate (H₂PO₄⁻) and hydronium ions (H₃O⁺) from the acid, compensating for the additional acid added. This helps to maintain the pH of the solution despite the presence of the added acid.

On the other hand, when a base is added to the buffer system, it accepts protons from the buffer's weak acid. This leads to a decrease in the concentration of the weak acid and an increase in the concentration of its conjugate base. Consequently, the equilibrium shifts to the right, favoring the production of more hydrogen phosphate (HPO₄²⁻) and hydroxide ions (OH⁻) from the base, offsetting the presence of the added base. This helps to keep the pH of the solution stable despite the presence of the added base.

In summary, when an acid is added to a buffer system, the equilibrium shifts to the left, while the addition of a base shifts the equilibrium to the right. This helps buffers to maintain a relatively constant pH in a given solution.