Introduction:
When making an aqueous solution of ferric chloride (FeCl3) in the laboratory, it is common to add hydrochloric acid (HCl) as well. This addition serves several important purposes in the preparation process. In this response, we will discuss the reasons behind the addition of HCl while making an aqueous solution of ferric chloride.

1. Dissociation of FeCl3:
One of the primary reasons for adding HCl is to aid in the dissociation of ferric chloride. FeCl3 is a salt that ionizes in water, forming Fe3+ cations and Cl- anions. However, this dissociation process can be slow and incomplete. By adding HCl, which is a strong acid, the H+ ions from the acid can react with the Cl- ions, converting them into HCl molecules. This reaction helps to increase the concentration of Fe3+ ions in the solution.

2. Solubility of FeCl3:
Ferric chloride is sparingly soluble in water, meaning that it does not readily dissolve in water. However, when HCl is added, it interacts with the FeCl3 to form a complex ion called hexaaquachloroferrate(III) ion [Fe(H2O)6]3+. This complex ion has a higher solubility in water compared to FeCl3 alone. Thus, the addition of HCl improves the solubility of ferric chloride, allowing for the preparation of a more concentrated and stable solution.

3. pH Adjustment:
HCl is a strong acid with a low pH, whereas ferric chloride is slightly acidic. By adding HCl, the overall pH of the solution can be adjusted to a desired level. This adjustment is important because it can impact the stability and reactivity of the solution. In certain applications, such as in wastewater treatment or etching processes, maintaining a specific pH range is crucial for the desired outcome.

4. Contaminant Control:
During the manufacturing or storage of ferric chloride, impurities may be introduced or formed, such as iron hydroxides or iron oxides. These impurities can adversely affect the quality and performance of the ferric chloride solution. However, by adding HCl, these impurities can be dissolved or prevented from forming. The acidic environment created by HCl helps to maintain the stability and purity of the solution.

Conclusion:
In summary, the addition of HCl while making an aqueous solution of ferric chloride serves multiple purposes. It aids in the dissociation of FeCl3, improves the solubility of the salt, adjusts the pH to a desired range, and helps control impurities. These factors contribute to the preparation of a more concentrated and stable ferric chloride solution, which is essential for various laboratory and industrial applications.

Aq. solution of ferric chloride gives ppt of ferric hydroxide and hydro chloric acid

FeCl3 + 3H2O ---> Fe(OH)3 + 3HCl

This solution is acid salt of weak base and strong acid