chemistry : class12 Phosphine generator apparatus : Preparation of phosphine : p-Block Elements MCQ Questions and Answers
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Phosphine generator apparatus
MCQ questions and answers.
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MCQ Questions
1. In a laboratory phosphine generator using phosphorus and concentrated aqueous sodium hydroxide, why is the phosphorus placed in the generator flask before the alkali is admitted through a thistle funnel?
A.
To ensure that the reaction begins only after the apparatus has been assembled and tested for leakage
B.
To prevent phosphorus from dissolving in the delivery tube before gas evolution begins
C.
To increase the solubility of phosphine in the reaction mixture
D.
To ensure that hydrogen, rather than phosphine, is evolved initially
Correct Answer:
To ensure that the reaction begins only after the apparatus has been assembled and tested for leakage
Explanation:
Phosphine generation starts when phosphorus comes into contact with hot concentrated alkali. Keeping the alkali in the thistle funnel until assembly and leak testing are complete prevents premature gas evolution and allows the apparatus to be safely purged.
2. Which arrangement is most appropriate for a phosphine generator in which phosphorus reacts with aqueous sodium hydroxide?
A.
A sealed flask containing phosphorus and alkali, fitted with a thistle funnel for controlled addition and a separate outlet for gas removal
B.
A completely sealed flask with no gas outlet, so that pressure forces phosphine into the receiver
C.
An open beaker containing phosphorus and alkali, with the delivery tube dipping directly into the liquid
D.
A flask having only a gas inlet, because phosphine is collected by displacement of sodium hydroxide
Correct Answer:
A sealed flask containing phosphorus and alkali, fitted with a thistle funnel for controlled addition and a separate outlet for gas removal
Explanation:
The generator must permit controlled addition of alkali while providing a separate path for phosphine to leave. It is closed enough to direct the gas through the purification and collection system, but it cannot be completely sealed without a gas outlet.
3. A phosphine generator is fitted with a thistle funnel whose lower end is not immersed below the liquid level in the generator flask. What is the principal operational defect?
A.
Gas can escape through the thistle funnel instead of being directed through the delivery line
B.
The sodium hydroxide becomes too dilute because phosphine enters the funnel
C.
The phosphorus is converted directly into phosphoric acid
D.
The delivery tube becomes filled with oxygen from the atmosphere
Correct Answer:
Gas can escape through the thistle funnel instead of being directed through the delivery line
Explanation:
The liquid seal at the lower end of the thistle funnel prevents phosphine from escaping through the addition tube. If the tip is above the liquid, the generator is not effectively closed and gas may leak through that path.
4. Why should a wash bottle or suitable purification stage be connected between the phosphine generator and the final collection vessel?
A.
The gas leaving the generator may contain alkaline mist and volatile impurities that should not enter the collection vessel
B.
The wash bottle converts phosphine completely into phosphorus pentoxide before collection
C.
The wash bottle supplies oxygen required for phosphine formation
D.
The wash bottle prevents the phosphorus pieces from contacting sodium hydroxide
Correct Answer:
The gas leaving the generator may contain alkaline mist and volatile impurities that should not enter the collection vessel
Explanation:
Gas from the generator can carry droplets of concentrated alkali and traces of other gaseous impurities. A properly selected washing stage removes such contaminants before phosphine is led to the receiver.
5. In assembling a phosphine generator, why should the delivery tube not terminate below the liquid level inside the generator flask?
A.
It could cause the liquid to be forced backward through the delivery line when gas evolution decreases
B.
It would prevent the thistle funnel from acting as a liquid seal
C.
It would stop phosphorus from reacting with sodium hydroxide by removing all water
D.
It would cause phosphine to decompose into phosphorus and hydrogen inside the flask
Correct Answer:
It could cause the liquid to be forced backward through the delivery line when gas evolution decreases
Explanation:
A delivery tube dipping into the generator liquid can permit suck-back when the apparatus cools or gas evolution slows. This may contaminate the purification train or create a hazardous pressure imbalance, so the gas outlet should remain above the liquid.
6. Before introducing the first portion of phosphine into a collection vessel, the generator and delivery system should be allowed to discharge gas for a short time mainly to:
A.
Replace the air initially present in the apparatus with the generated gas
B.
Convert the sodium hydroxide into sodium phosphite
C.
Remove all water from the phosphorus surface
D.
Increase the density of phosphine by compressing it in the generator
Correct Answer:
Replace the air initially present in the apparatus with the generated gas
Explanation:
The assembled apparatus initially contains air. Discharging the first gas through the outlet purges much of this air, reducing the formation of a potentially dangerous phosphine–air mixture in the receiver.
7. A pressure-equalizing feature is useful in a phosphine generator because it primarily:
A.
Prevents pressure changes from forcing reaction liquid backward through the addition tube
B.
Ensures that phosphine is oxidized before entering the delivery tube
C.
Keeps the phosphorus permanently suspended above the alkali
D.
Raises the boiling point of the sodium hydroxide solution
Correct Answer:
Prevents pressure changes from forcing reaction liquid backward through the addition tube
Explanation:
Gas evolution and cooling can produce pressure fluctuations within the generator. Pressure equalization, together with a properly immersed addition tube, helps prevent backflow and allows controlled operation of the apparatus.
8. If the joints of a phosphine generator are not gas-tight, which observation most directly indicates the defect during operation?
A.
Gas evolution appears weak at the outlet even though the phosphorus and alkali are reacting
B.
The sodium hydroxide solution becomes colourless immediately
C.
The phosphorus settles faster because its density has increased
D.
The thistle funnel becomes filled with solid sodium hydroxide
Correct Answer:
Gas evolution appears weak at the outlet even though the phosphorus and alkali are reacting
Explanation:
A leak allows phosphine to escape before reaching the delivery outlet, so the observed gas flow at the intended outlet is lower than expected despite ongoing reaction in the generator.
9. Which modification best permits the rate of phosphine generation to be controlled in a phosphorus–alkali generator?
A.
Adding the concentrated alkali gradually through a stoppered thistle funnel
B.
Sealing the delivery tube completely after the first bubbles appear
C.
Filling the generator flask completely with alkali before inserting phosphorus
D.
Replacing the gas outlet with a second solid stopper
Correct Answer:
Adding the concentrated alkali gradually through a stoppered thistle funnel
Explanation:
Controlled addition of alkali regulates the area and rate at which phosphorus contacts the reactive solution. A stoppered thistle funnel also helps maintain a closed gas path while the reagent is added.
10. Why is the receiver in a phosphine-generating setup normally connected only after the delivery line has been checked for continuity and leakage?
A.
A faulty line could allow phosphine to escape into the laboratory or permit air to remain mixed with the collected gas
B.
The receiver must first be filled with concentrated sodium hydroxide to activate phosphine
C.
Phosphine can be collected only after phosphorus has been removed from the generator
D.
Checking the line converts the gas into a liquid suitable for collection
Correct Answer:
A faulty line could allow phosphine to escape into the laboratory or permit air to remain mixed with the collected gas
Explanation:
Leak testing confirms that the gas will follow the intended route from generator through the purification stage to the receiver. It also helps minimize the hazardous retention or leakage of phosphine–air mixtures.
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