To calculate the strength of phosphoric acid (H3PO4) from the strength of its anhydride, phosphorus pentoxide (P2O5), you need to take into account the stoichiometry of the reaction between P2O5 and water (H2O). Phosphorus pentoxide reacts with water to form phosphoric acid. The balanced chemical equation for this reaction is as follows:
P2O5 + 3H2O → 2H3PO4
From this equation, you can see that one mole of P2O5 reacts with three moles of water to produce two moles of H3PO4. Therefore, you can calculate the strength (concentration) of phosphoric acid (H3PO4) from the strength of phosphorus pentoxide (P2O5) using the following formula:
Strength of H3PO4 (in g/L) = (Strength of P2O5 × Molecular weight of P2O5) / (2 × Molecular weight of H3PO4)
Where:
- Strength of P2O5 is the strength of phosphorus pentoxide in g/L.
- Molecular weight of P2O5 is the molar mass of phosphorus pentoxide, which is approximately 141.94 g/mol.
- Molecular weight of H3PO4 is the molar mass of phosphoric acid, which is approximately 98.00 g/mol.
Let’s say you have a solution of phosphorus pentoxide with a strength of 100 g/L. You can calculate the corresponding strength of phosphoric acid as follows:
Strength of H3PO4 (in g/L) = (100 g/L × 141.94 g/mol) / (2 × 98.00 g/mol) ≈ 71.97 g/L
So, the strength of the resulting phosphoric acid solution is approximately 71.97 g/L.
This calculation is important when you need to prepare or dilute phosphoric acid solutions from phosphorus pentoxide or when you want to determine the concentration of phosphoric acid in a solution based on the known concentration of P2O5.
