Question

A 19.3−g sample of white phosphorus was burned in excess of oxygen. The product was dissolved in enough water to make 504 mL of solution. Calculate the pH of the solution at 25°C.

pH =

Answer 1

P₄ + 5O₂ ---------> P₄O₁₀

P₄O₁₀ + 6H₂O -----------> 4 H₃PO₄

Molecular weight of P₄ = 123.89 g/mol

Amount of white phosphorus reacted = 19.3g

No. of moles of white phosphorus = 19.3 g / 123.89 gmol^-1 = 0.1157 mol

one mole of P₄O₁₀ yields 4 moles of H₃PO₄

so, 0.1157 moles of P₄O₁₀ yields 0.6231 moles of H₃PO₄

0.6231 moles of H₃PO₄ is dissolved in 0.504 L of water

So, the concentration of H₃PO₄ is : 0.6231 moles / 0.504 L = 1.236 mol L^-1

pH = -log [H⁺] = -log [1.236] = -0.092

pH = -0.092