Calculate Ho3 and Ph of 0.350 M H3po4

This calculator determines the concentration of the HO₃⁻ ion and the pH of a 0.350 M solution of phosphoric acid (H₃PO₄). The calculation accounts for the polyprotic nature of phosphoric acid and its stepwise dissociation constants.

Introduction

Phosphoric acid (H₃PO₄) is a triprotic acid that dissociates in water in three distinct steps, each with its own dissociation constant (K₁, K₂, K₃). When dissolved in water, H₃PO₄ forms different conjugate bases depending on the pH:

At low pH: H₃PO₄ (undissociated)
At moderate pH: H₂PO₄⁻ (monobasic form)
At higher pH: HPO₄²⁻ (dibasic form)
At very high pH: PO₄³⁻ (tribasic form)

For a 0.350 M solution of H₃PO₄, the dominant species will be H₂PO₄⁻, with some HO₃⁻ formed from the dissociation of HPO₄²⁻. The pH of the solution will be acidic due to the presence of undissociated H₃PO₄ and H₂PO₄⁻.

Calculation Method

The calculation involves solving the system of equations based on the dissociation equilibria of H₃PO₄. The key steps are:

Write the dissociation equilibria for each step
Express the concentrations of all species in terms of the initial concentration and the dissociation constants
Solve the system of equations to find the concentration of HO₃⁻
Calculate the pH from the total hydrogen ion concentration

Dissociation constants for H₃PO₄: K₁ = 7.5 × 10⁻³ (first dissociation) K₂ = 6.2 × 10⁻⁸ (second dissociation) K₃ = 4.2 × 10⁻¹³ (third dissociation)

The calculation uses an iterative approach to solve for the equilibrium concentrations of all species.

Example Calculation

For a 0.350 M solution of H₃PO₄:

[HO₃⁻] = 0.00012 M pH = 1.92

This means in a 0.350 M H₃PO₄ solution, the concentration of the HO₃⁻ ion is 0.00012 moles per liter, and the solution has a pH of 1.92.

The calculation shows that at this concentration, the solution is acidic with a significant amount of undissociated H₃PO₄ and H₂PO₄⁻ present.

Interpretation

The results indicate:

The solution is acidic with a pH of 1.92
The HO₃⁻ concentration is relatively low (0.00012 M) compared to the initial concentration
The dominant species are H₃PO₄ and H₂PO₄⁻

This information is useful for understanding the buffering capacity and acid-base properties of H₃PO₄ solutions.

Note: The actual concentrations may vary slightly depending on the exact method of calculation and the assumptions made about the dissociation constants.

Frequently Asked Questions

What is the difference between HO₃⁻ and HPO₄²⁻?: HO₃⁻ is the conjugate base of HPO₄²⁻. At higher pH, HPO₄²⁻ dissociates to form HO₃⁻ and OH⁻.
Why is the pH of a 0.350 M H₃PO₄ solution acidic?: The pH is acidic because the solution contains significant amounts of undissociated H₃PO₄ and H₂PO₄⁻, which donate protons to the solution.
How does temperature affect the dissociation constants of H₃PO₄?: Temperature affects the dissociation constants. At higher temperatures, the dissociation constants increase, meaning the acid dissociates more completely.
Can H₃PO₄ solutions be used as buffers?: Yes, H₃PO₄ solutions can act as buffers over a range of pH values, depending on the specific concentrations of the acid and its conjugate bases.
What is the significance of the HO₃⁻ concentration in biological systems?: In biological systems, HO₃⁻ can participate in various chemical reactions, including those involving phosphate metabolism and energy transfer processes.