To calculate the pH of a solution with a hydrogen ion concentration of 4.2 x 10^-5 M, we can use the pH formula:pH = -log10[H+]where [H+] is the hydrogen ion concentration.pH = -log10 4.2 x 10^-5 pH 4.38The theoretical pH of the solution is approximately 4.38.To determine the pH of the solution using a pH meter, follow these steps:1. Calibrate the pH meter using standard buffer solutions with known pH values e.g., pH 4, 7, and 10 .2. Rinse the electrode with distilled water and gently blot dry with a lint-free tissue.3. Immerse the electrode in the solution with the hydrogen ion concentration of 4.2 x 10^-5 M.4. Wait for the pH meter reading to stabilize, and record the pH value displayed.After measuring the pH with the pH meter, compare the experimental value to the theoretical value calculated using the Henderson-Hasselbalch equation. Note that the Henderson-Hasselbalch equation is used for calculating the pH of a buffer solution, which involves a weak acid and its conjugate base or a weak base and its conjugate acid. In this case, we are only given the hydrogen ion concentration, so the pH formula is more appropriate for calculating the theoretical pH value.