chemistry : class12 Diphosphane impurity : Preparation of phosphine : p-Block Elements MCQ Questions and Answers
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Diphosphane impurity
MCQ questions and answers.
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MCQ Questions
1. Phosphine prepared by treating calcium phosphide with water may ignite spontaneously in air mainly because of the presence of which impurity?
A.
P4O6
B.
P2H4
C.
H3PO3
D.
PH2OH
Correct Answer:
P2H4
Explanation:
Diphosphane, P2H4, is formed as a minor impurity during phosphine preparation and is responsible for the spontaneous inflammability of the impure gas in air.
2. Which statement most accurately distinguishes diphosphane from phosphine in freshly prepared laboratory phosphine?
A.
Diphosphane is the principal product, whereas phosphine is only a trace impurity
B.
Diphosphane is responsible for spontaneous ignition, whereas pure phosphine is not spontaneously inflammable in air
C.
Diphosphane prevents phosphine from burning in air
D.
Diphosphane is formed only after phosphine has been completely oxidised
Correct Answer:
Diphosphane is responsible for spontaneous ignition, whereas pure phosphine is not spontaneously inflammable in air
Explanation:
The freshly generated gas contains traces of P2H4. These traces cause spontaneous ignition in air; pure PH3 itself does not account for this characteristic.
3. A sample of impure phosphine contains 0.010 mol of diphosphane. Assuming complete combustion according to P2H4 + 4O2 → P2O5 + 2H2O, how much oxygen is consumed by the diphosphane alone?
A.
0.010 mol
B.
0.020 mol
C.
0.040 mol
D.
0.080 mol
Correct Answer:
0.040 mol
Explanation:
The balanced combustion equation requires 4 mol of O2 per mol of P2H4. Therefore, 0.010 mol of P2H4 consumes 0.010 × 4 = 0.040 mol of oxygen.
4. Which molecular formula and phosphorus-to-hydrogen atomic ratio correspond to the diphosphane impurity in impure phosphine?
A.
PH3 and 1:3
B.
P2H2 and 1:1
C.
P2H4 and 1:2
D.
P4H6 and 2:3
Correct Answer:
P2H4 and 1:2
Explanation:
Diphosphane has the molecular formula P2H4. Its phosphorus-to-hydrogen ratio is therefore 2:4, which simplifies to 1:2.
5. Why does a jet of freshly prepared impure phosphine sometimes produce visible smoke or flame on contact with air, whereas purified phosphine does not show the same spontaneous behaviour?
A.
The gas contains dissolved phosphorus pentoxide, which decomposes in air
B.
Trace diphosphane undergoes rapid oxidation and initiates ignition
C.
Pure phosphine is non-combustible, but diphosphane is explosive only in water
D.
Calcium phosphide vapour reacts directly with atmospheric nitrogen
Correct Answer:
Trace diphosphane undergoes rapid oxidation and initiates ignition
Explanation:
Diphosphane is much more readily oxidised than phosphine. Its rapid oxidation in air causes the characteristic spontaneous ignition of impure phosphine.
6. A gaseous mixture contains x mol of PH3 and y mol of P2H4. If complete combustion is represented by 2PH3 + 4O2 → P2O5 + 3H2O and P2H4 + 4O2 → P2O5 + 2H2O, what is the total oxygen requirement?
A.
2x + 4y mol
B.
4x + 2y mol
C.
2x + 2y mol
D.
4x + 4y mol
Correct Answer:
2x + 4y mol
Explanation:
Each mole of PH3 requires 2 mol of O2, while each mole of P2H4 requires 4 mol of O2. Thus, the total requirement is 2x + 4y mol.
7. In a sample of impure phosphine, the mole fraction of diphosphane is 0.020. If the total sample contains 0.500 mol of gas, how many moles of diphosphane are present?
A.
0.0010 mol
B.
0.010 mol
C.
0.020 mol
D.
0.025 mol
Correct Answer:
0.010 mol
Explanation:
Moles of diphosphane = mole fraction × total moles = 0.020 × 0.500 = 0.010 mol. This trace quantity is sufficient to make the phosphine spontaneously inflammable.
8. Which observation is most specifically associated with the presence of diphosphane in laboratory-prepared phosphine?
A.
The gas dissolves completely in water without reaction
B.
The gas becomes non-combustible in excess oxygen
C.
The gas may catch fire spontaneously on exposure to air
D.
The gas changes into nitrogen dioxide in darkness
Correct Answer:
The gas may catch fire spontaneously on exposure to air
Explanation:
Spontaneous ignition on exposure to air is the characteristic indication of diphosphane contamination in freshly prepared phosphine.
9. For equal amounts of phosphorus atoms present, which comparison between PH3 and P2H4 is correct regarding hydrogen atoms?
A.
PH3 contains twice as many hydrogen atoms as P2H4
B.
P2H4 contains twice as many hydrogen atoms as PH3
C.
Both contain the same number of hydrogen atoms
D.
P2H4 contains three times as many hydrogen atoms as PH3
Correct Answer:
P2H4 contains twice as many hydrogen atoms as PH3
Explanation:
For one phosphorus atom, PH3 contains 3 hydrogen atoms, whereas P2H4 contains 4 hydrogen atoms for 2 phosphorus atoms, or 2 hydrogen atoms per phosphorus atom. Thus, for equal phosphorus amounts, PH3 actually contains more hydrogen atoms, so the correct comparison is not listed as stated; therefore this question is invalid.
10. A sample of impure phosphine contains 0.020 mol each of PH3 and P2H4. Using the balanced combustion equations for both gases, how many moles of O2 are required for complete combustion?
A.
0.080 mol
B.
0.100 mol
C.
0.120 mol
D.
0.160 mol
Correct Answer:
0.120 mol
Explanation:
The PH3 portion requires 0.020 × 2 = 0.040 mol O2, and the P2H4 portion requires 0.020 × 4 = 0.080 mol O2. The total is 0.120 mol O2.
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