1. The problem statement, all variables and given/known data Why does a 12mol/L solution of HCl used to clean rust off iron get a brown coating after being exposed to air and washed with water, while the same concentration solution of phosphoric acid takes a day to have an apparent brown coating. 2. Relevant equations Explain the similarities and differences in the chemistry of each acid when used to remove rust. 3. The attempt at a solution 6HCl+ Fe2O3 →2FeCl3 + 3H2O 2H3PO4 + Fe2O3 → 2FePO4 + 3H2O Phosphoric is a weak acid? Lower concentration of hydrogen ions.