When a sample of air is passed through alkaline pyrogalol, potash and finally through U-tube containing fused…

To solve this question, we need to understand the chemical process of removing specific components from a sample of air. Let’s break down the function of each substance used in the experiment:

1. Analyze the components of air

Air is a mixture primarily composed of nitrogen (~78%), oxygen (~21%), noble gases (~0.9%), and carbon(IV) oxide (~0.04%).

2. Explain the chemical processes

• Alkaline Pyrogallol: This is a strong reducing agent used specifically to absorb oxygen from the air.
• Potash (Potassium Hydroxide, KOH): This is a strong base used to absorb acidic gases, specifically carbon(IV) oxide ($CO_2$).
• Fused Calcium Chloride ($CaCl_2$): This is a drying agent (desiccant) used to absorb water vapor (moisture) from the air.

3. Step-by-step solution

• Step 1: The air passes through alkaline pyrogallol, which removes the oxygen.
• Step 2: The remaining air passes through potash, which removes the carbon(IV) oxide.
• Step 3: The air then passes through the U-tube containing fused calcium chloride, which removes the water vapor.

After these three substances have done their work, the components that were not absorbed are the ones that do not react with these chemicals. These are:

• Nitrogen: It is chemically inert under these conditions and does not react with pyrogallol, KOH, or $CaCl_2$.
• Noble Gases: These are chemically inert (unreactive) and will pass through all the reagents unchanged.

4. Conclusion

Since oxygen, carbon(IV) oxide, and water vapor have been removed, the remaining components are nitrogen and the noble gases.

Comparing this to your options:

• Option A is incorrect because $CO_2$ was removed by the potash.
• Option B is incorrect because $CO_2$ was removed by the potash.
• Option C is correct because both nitrogen and noble gases remain unabsorbed.
• Option D is incorrect because $CO_2$ was removed.

Exam Tip: Remember the "Order of Removal" mnemonic: P-O-W (Pyrogallol for Oxygen, Potash for Water-gas/$CO_2$, Water-absorber for moisture). Always check if the question asks what is removed or what is left behind to avoid silly mistakes!