Question

a) Write the formula of the hydronium ion. How is the concentration of hydronium ions affected when a solution of an acid is diluted?
 b) In between ethanoic acid and hydrochloric acid which one will produce more hydronium ions in their respective aqueous solution?

Answer 1

Hint: The hydronium ion is written as${{H}_{3}}{{O}^{+}}$. The strength of the solution can be determined by concentration of hydronium ions. On diluting an acid the concentration of hydronium ions decreases. The hydronium ion concentration can be found from the pH by the reverse of the mathematical operation employed to find the pH.

Complete answer:
a) The formula for hydronium ion is ${{H}_{3}}{{O}^{+}}$.The concentration of hydronium ions decreases when an acid is diluted because on adding water the ${H}^{+}$ ions of the acid and hydroxyl ions of water react to form water molecules and the concentration of hydronium ions decreases. A given solution of an acid is diluted by adding more water. Thus, the overall volume of the solution increases. However, the number of hydrogen ions in solution remains the same. Therefore, the number of hydrogen ions per unit volume decreases and hence the concentration of hydronium ions decreases.
$2{{H}_{2}}{{O}_{(l)}}\to O{{H}_{aq}}^{-}+{{H}_{3}}O_{aq}^{+}$
b) A weak acid is one which doesn't ionize fully when it is dissolved in water. Ethanoic acid is a typical weak acid. It reacts with water to produce hydronium ions and ethanoate ions, but the back reaction is more successful than the forward one. The ions react very easily to reform the acid and the water. At any one time, only about 1% of the ethanoic acid molecules have converted into ions. The rest remain as simple ethanoic acid molecules.
$C{{H}_{3}}COOH+{{H}_{2}}O\to C{{H}_{3}}CO{{O}^{-}}+{{H}_{3}}{{O}^{+}}$
When hydrochloric acid dissolves in water, it ionizes, splitting up into hydrogen ${H}^{+}$ and chlorine ${Cl}^{-}$ ions. Hydrogen ions ${H}^{+}$ are attracted to the negative oxygen end of a water molecule, combining to form hydronium ions. When an acid dissolves in water, a proton (hydrogen ion) is transferred to a water molecule to produce a hydronium ion. Hydrochloric acid is a strong acid so it dissociates completely into water. Therefore, HCl produces more hydronium ions in aqueous solution as compared to ethanoic acid.
$HC{{l}_{g}}+{{H}_{2}}{{O}_{l}}\to {{H}_{3}}{{O}_{aq}}^{+}+Cl {aq}^{-}$

Note:
The higher the \[{{K}_{a}}\] value, the greater the no. of hydrogen ions liberated per mole of acid in the solution and hence stronger is the acid. The more easily the acid dissociates, and the stronger it is and more it releases the hydronium ions. The ionization constant for ethanoic acid is less than hydrochloric acid which means it is weaker than hydrochloric acid.