The impurities formed during the laboratory preparation of chlorine gas are removed by

Analysis

In the laboratory preparation of chlorine gas ($Cl_2$), concentrated hydrochloric acid ($HCl$) is reacted with an oxidizing agent like manganese(IV) oxide ($MnO_2$). The resulting gas is typically contaminated with hydrogen chloride gas ($HCl$) and water vapor ($H_2O$). To obtain pure, dry chlorine gas, these impurities must be removed.

Concept

To purify a gas, we use "scrubbing" agents:

1. Removal of $HCl$ gas: The gas is passed through water ($H_2O$). Since $HCl$ is extremely soluble in water, it dissolves, leaving the chlorine gas behind.
2. Removal of water vapor: The gas is then passed through concentrated tetraoxosulphate(VI) acid ($H_2SO_4$). Concentrated $H_2SO_4$ is a powerful dehydrating agent that absorbs the remaining moisture.

Step-by-Step Solution

1. Identify the impurities: The gas produced ($Cl_2$) is mixed with $HCl$ (from the acid used) and water vapor (from the reaction).
2. First purification stage: The mixture is bubbled through a wash bottle containing water. This removes the $HCl$ gas because $HCl$ is highly soluble in water, whereas $Cl_2$ is only sparingly soluble.
3. Second purification stage: The gas is then passed through a drying tower containing concentrated $H_2SO_4$. This removes the water vapor.
4. Evaluating the options: The question asks for the substance used to remove the impurities (specifically referring to the initial wash). Water is the primary agent used to wash away the $HCl$ impurity.

Conclusion

While $H_2SO_4$ is used to dry the gas, water is the specific reagent used to remove the $HCl$ impurity formed during the preparation.

Exam Tip

Remember the "Wash and Dry" sequence for chlorine: Water to wash away $HCl$, then Conc. $H_2SO_4$ to dry it. If the question asks what is used to dry the gas, choose $H_2SO_4$. If it asks what is used to remove impurities (referring to the acid fumes), choose $H_2O$.