Final answer:
An ICE table is used to calculate the equilibrium concentrations for the reaction of formic acid (HCHO₂) and its conjugate base (CHO₂⁻) within a buffer solution.
Step-by-step explanation:
To solve this equilibrium problem and calculate the pH of the given solution, we will use an ICE table (which stands for Initial, Change, Equilibrium). The solution contains a weak acid, formic acid (HCHO₂), and its sodium salt (NaCHO₂), forming a buffer solution.
The equilibrium reaction is:
HCHO₂(aq) + H₂O(l) <=> H₃O⁺(aq) + CHO₂⁻(aq)
We start with the initial concentrations ([HCHO₂] = 0.15 M and [NaCHO₂] = 0.12 M). An ICE table is used to account for the changes as the reaction goes to equilibrium. Since NaCHO₂ dissociates completely in water, it provides the initial concentration of CHO₂⁻ ions. The acid dissociation constant (Ka) for formic acid is needed for calculating the concentrations at equilibrium. Let's assume the Ka value for HCHO₂ is approximately 1.8 x 10⁻´.
Initially, the concentration of H₃O⁺ is negligible. As the equilibrium establishes, x amount of HCHO₂ disassociates to give x amount of H₃O⁺ and x amount of CHO₂⁻. Because NaCHO₂ is already in the ion form, its concentration decreases by x amount. Once we have 'x', the concentration of H₃O⁺, we can calculate pH by taking the negative logarithm (-log[H₃O⁺]).
Since the system is a buffer, we can also use the Henderson-Hasselbalch equation to find pH, which is simpler with the given concentrations. The equation is: pH = pKa + log([A⁻]/[HA]), where [A⁻] is the concentration of the base form (CHO₂⁻), and [HA] is the concentration of the acid form (HCHO₂).