Why can you say in: HCl + H2O -> Cl- + H3O, that the moles of HCl is equal to moles of H30+?

1 Answer
Feb 14, 2018

Why? Because hydrogen chloride is very soluble in solution, and in solution undergoes almost complete protonolysis.

Explanation:

...hydrogen chloride undergoes the protonolysis reaction...

#HCl(aq) rightleftharpoons H_3O^+ + Cl^-#

...and this equilibrium lies strongly to the right as we face the page.

And given a starting mass of hydrogen chloride gas, we have an equivalent quantity of hydronium and chloride ions in solution, right up to the maximum concentration of hydrochloric acid, approx. #10.6*mol*L^-1# under standard conditions.

On the other hand, for weaker acids, say hydrogen fluoride, the equilibrium would lie somewhat to the left and #[H_3O^+]!=[HF]#.