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Calculate Each of The Following in 2.50 Mol of H3po4

Reviewed by Calculator Editorial Team

This calculator helps determine the moles of H+, PO4^3-, H2PO4-, and H3PO4 in a 2.50 mol solution of H3PO4. Understanding these values is essential for chemistry students and professionals working with phosphoric acid solutions.

Introduction

Phosphoric acid (H3PO4) is a polyprotic acid that can donate up to three protons. When dissolved in water, it exists in several forms depending on the pH of the solution. Calculating the moles of each species helps in understanding the acid-base properties and behavior of H3PO4 solutions.

This guide explains how to calculate the moles of H+, PO4^3-, H2PO4-, and H3PO4 in a given amount of H3PO4, using the equilibrium constants for each dissociation step.

Calculation Method

The calculation involves determining the distribution of H3PO4 species based on the pH of the solution. The general approach is:

  1. Assume the initial moles of H3PO4 are all undissociated.
  2. Calculate the moles of H+ and PO4^3- based on the first dissociation.
  3. Calculate the moles of H2PO4- based on the second dissociation.
  4. Calculate the moles of H3PO4 based on the third dissociation.

Key Formulas

The dissociation constants for H3PO4 are:

  • Ka1 = 7.5 × 10⁻³ (first dissociation)
  • Ka2 = 6.2 × 10⁻⁸ (second dissociation)
  • Ka3 = 4.2 × 10⁻¹³ (third dissociation)

The general formula for calculating the moles of each species is based on the equilibrium concentrations and the initial moles of H3PO4.

Example Calculation

For a 2.50 mol solution of H3PO4:

  1. Assume all 2.50 mol are initially H3PO4.
  2. Calculate the moles of H+ and PO4^3- using Ka1.
  3. Calculate the moles of H2PO4- using Ka2.
  4. Calculate the moles of H3PO4 using Ka3.

Note: The exact values depend on the pH of the solution. For a neutral solution, the distribution is approximately 2.50 mol H3PO4, 0 mol H2PO4-, 0 mol H+, and 0 mol PO4^3-.

Common Questions

What is the difference between H3PO4, H2PO4-, and PO4^3-?

H3PO4 is the undissociated form of phosphoric acid. H2PO4- is the monovalent anion formed after the first dissociation. PO4^3- is the trivalent anion formed after the second dissociation.

How does pH affect the distribution of H3PO4 species?

The pH determines which dissociation steps occur. At low pH, more H3PO4 remains undissociated. At higher pH, more H2PO4- and PO4^3- are formed.