chemistry : class12 Phosphine collection and safety : Preparation of phosphine : p-Block Elements MCQ Questions and Answers
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Phosphine collection and safety
MCQ questions and answers.
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MCQ Questions
1. Which collection method is most appropriate for phosphine in the laboratory, considering both its water solubility and safety?
A.
Collection over water because phosphine is only slightly soluble in water
B.
Collection by upward displacement of air because phosphine is highly soluble in water
C.
Collection by downward displacement of air because phosphine is lighter than air
D.
Collection over concentrated sulfuric acid because phosphine reacts with water
Correct Answer:
Collection over water because phosphine is only slightly soluble in water
Explanation:
Phosphine is only slightly soluble in water, so it can be collected by displacement of water. Collection over air is less desirable because phosphine is poisonous and inflammable.
2. Why is phosphine generally not collected by directly displacing air in an open laboratory setup?
A.
It rapidly polymerizes on contact with nitrogen
B.
It is poisonous and forms a potentially inflammable mixture with air
C.
It is denser than water and sinks through the collection vessel
D.
It reacts explosively with oxygen-free air
Correct Answer:
It is poisonous and forms a potentially inflammable mixture with air
Explanation:
Phosphine is highly poisonous, and its contact with air can create a hazardous inflammable mixture. Therefore, collection over water in a properly controlled apparatus is safer than open air displacement.
3. A sample of phosphine prepared in the laboratory catches fire spontaneously when it comes into contact with air. Which impurity most plausibly accounts for this behavior?
A.
Phosphorus trichloride
B.
Phosphorus pentoxide
C.
Diphosphine, P2H4
D.
Phosphoric acid
Correct Answer:
Diphosphine, P2H4
Explanation:
Impure phosphine may contain diphosphine, P2H4, which is spontaneously inflammable in air. This impurity is responsible for the characteristic spontaneous ignition of freshly prepared impure phosphine.
4. Why is phosphine collected over water rather than over an aqueous oxidizing solution?
A.
Oxidizing solutions increase the vapor pressure of water
B.
Phosphine can be oxidized by the oxidizing solution, causing loss of the gas and danger
C.
Phosphine becomes completely insoluble only in oxidizing solutions
D.
Oxidizing solutions convert phosphine into a non-toxic inert gas without reaction
Correct Answer:
Phosphine can be oxidized by the oxidizing solution, causing loss of the gas and danger
Explanation:
Phosphine is a reducing substance and can be oxidized by suitable oxidizing agents. Hence, an oxidizing collection liquid could consume the gas and create additional safety hazards; water is preferred.
5. Which precaution is most important when transferring a bottle of collected phosphine from a water trough?
A.
Expose the bottle mouth to air for rapid ventilation
B.
Keep the gas away from flames and prevent uncontrolled mixing with air
C.
Shake the bottle vigorously to remove dissolved water
D.
Add concentrated nitric acid to stabilize the gas
Correct Answer:
Keep the gas away from flames and prevent uncontrolled mixing with air
Explanation:
Phosphine is poisonous and inflammable, while impure phosphine may ignite spontaneously because of diphosphine. Contact with flames and uncontrolled air mixing must therefore be avoided during transfer.
6. A delivery tube is used to collect phosphine over water. Which observation would most strongly indicate that air has not yet been completely displaced from the apparatus?
A.
The gas burns only after being mixed with oxygen
B.
The collected gas forms a potentially explosive mixture on ignition
C.
The gas remains slightly soluble in water
D.
The gas is collected in an inverted vessel
Correct Answer:
The collected gas forms a potentially explosive mixture on ignition
Explanation:
Residual air mixed with phosphine can produce a dangerous inflammable mixture. Complete displacement of air before testing or ignition is therefore an essential safety precaution.
7. Which combination correctly describes the two main hazards that govern the collection of phosphine?
A.
High corrosivity and complete nonflammability
B.
High toxicity and inflammability
C.
Radioactivity and explosive decomposition in water
D.
Strong alkalinity and spontaneous freezing
Correct Answer:
High toxicity and inflammability
Explanation:
Phosphine is a highly poisonous gas and is inflammable. These properties require collection in a closed apparatus, preferably over water, with exclusion of ignition sources and adequate ventilation.
8. Why should the first portion of gas evolved during phosphine preparation generally not be treated as pure phosphine for immediate ignition tests?
A.
It is likely to contain air from the apparatus, producing a hazardous mixture
B.
It contains only water vapor and cannot support any reaction
C.
It is always pure hydrogen because phosphine forms later
D.
It contains oxygen generated by decomposition of water
Correct Answer:
It is likely to contain air from the apparatus, producing a hazardous mixture
Explanation:
The apparatus initially contains air, which is expelled along with the first gas evolved. Igniting this mixture can be dangerous, so the gas should first be allowed to displace the air and should be handled in a controlled manner.
9. Which statement best explains why a phosphine collection vessel is commonly kept inverted over water during collection?
A.
Phosphine is completely insoluble in water and is denser than mercury
B.
Phosphine is slightly soluble in water, allowing water displacement to collect it with limited loss
C.
Phosphine reacts with water to form a protective solid film
D.
Phosphine is lighter than water vapor and therefore rises through water
Correct Answer:
Phosphine is slightly soluble in water, allowing water displacement to collect it with limited loss
Explanation:
The water-displacement arrangement permits collection because phosphine dissolves only slightly in water. It also reduces direct contact between the toxic, inflammable gas and atmospheric air.
10. A laboratory worker proposes collecting phosphine in a gas jar by downward displacement of air because phosphine has a molar mass greater than that of air. What is the best evaluation of this proposal?
A.
It is preferred because density alone eliminates the toxicity hazard
B.
It is unsuitable as the primary method because direct contact with air creates toxicity and inflammability risks
C.
It is impossible because phosphine is lighter than air
D.
It is preferred because phosphine is completely miscible with air but nonflammable
Correct Answer:
It is unsuitable as the primary method because direct contact with air creates toxicity and inflammability risks
Explanation:
Although phosphine is slightly denser than air, allowing consideration of downward air displacement, its poisonous and inflammable nature makes collection over water safer and more appropriate when feasible.
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