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Calculate Ph and H3o for 0.350 M H3po4 Solution

Reviewed by Calculator Editorial Team

This calculator determines the pH and hydronium ion concentration [H3O+] for a 0.350 M solution of phosphoric acid (H3PO4). The calculation accounts for the polyprotic nature of phosphoric acid and its dissociation steps.

Introduction

Phosphoric acid (H3PO4) is a triprotic acid that dissociates in three steps, each with different dissociation constants (Ka1, Ka2, Ka3). The pH of a solution depends on which dissociation steps are significant at the given concentration.

For a 0.350 M solution, the first dissociation step dominates, making the solution acidic with a pH less than 7. The calculator uses the dissociation constants and the initial concentration to compute the pH and [H3O+].

Calculation Method

The calculation follows these steps:

  1. Identify the dominant dissociation step based on the initial concentration.
  2. Use the appropriate dissociation constant for the dominant step.
  3. Calculate the equilibrium concentration of H3O+ using the dissociation constant.
  4. Convert the [H3O+] to pH using the equation pH = -log[H3O+].

Key Formulas

For the first dissociation step (dominant at 0.350 M):

Ka1 = [H3O+][H2PO4-]/[H3PO4]

At equilibrium, [H3O+] ≈ √(Ka1 × [H3PO4])

pH = -log[H3O+]

Example Calculation

For a 0.350 M H3PO4 solution:

  1. Use Ka1 = 7.5 × 10-3 for the first dissociation step.
  2. Calculate [H3O+] ≈ √(7.5 × 10-3 × 0.350) ≈ 0.0612 M.
  3. Convert to pH: pH = -log(0.0612) ≈ 1.22.

The actual calculation accounts for the initial concentration and dissociation constants more precisely, but this example illustrates the method.

Interpretation of Results

The pH of 1.22 indicates a strongly acidic solution. The [H3O+] of 0.0612 M shows significant hydronium ions, consistent with the acidic nature of the solution.

For comparison, a pH of 7 is neutral, pH less than 7 is acidic, and pH greater than 7 is basic. The results show that the solution is acidic due to the dissociation of H3PO4.

Frequently Asked Questions

Why is the pH of a 0.350 M H3PO4 solution less than 7?

The pH is less than 7 because H3PO4 is an acid that dissociates in water, releasing H3O+ ions and making the solution acidic.

How does the concentration of H3PO4 affect the pH?

Higher concentrations of H3PO4 lead to more dissociation and higher [H3O+], resulting in a lower pH.

Can I use this calculator for other concentrations of H3PO4?

Yes, the calculator can be used for any concentration of H3PO4, but the method changes for very dilute or very concentrated solutions.