Which gas diffuses slowest? MCQ with Answer and Explanation

Which gas diffuses slowest?
A. CO₂
B. NH₃
C. H₂
D. CH₄
Answer: Option A
Solution (By JKSSB Mock Tests)
Rate ∝ 1/√M. Molar masses: H₂=2, CH₄=16, NH₃=17, CO₂=44. CO₂ has highest mass, diffuses slowest.

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The allotrope of carbon used in electrodes and as a neutron moderator in nuclear reactors is:
A. Fullerene
B. Graphite
C. Charcoal
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Correct Answer: Option B


Explanation:
Graphite is used in electrodes (due to electrical conductivity) and as a moderator in nuclear reactors (to slow down neutrons). It is chemically stable and can withstand high temperatures. Diamond is an insulator. Fullerene is a nanomaterial. Charcoal is used as adsorbent and fuel. Graphite blocks are used in RBMK reactors (like Chernobyl) and in advanced gas-cooled reactors.

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Question #2
The valency of iron in Fe₂O₃ is:
A. 4
B. 2
C. 1
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Correct Answer: Option D


Explanation:
In Fe₂O₃, oxygen has valency 2. Two Fe atoms combined with three O atoms: total negative charge = 6. For two Fe, total positive = 6, so each Fe has +3. Valency = 3 (ferric). In FeO, valency is 2 (ferrous).

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Question #3
The chemical formula of plaster of Paris is:
A. CaO
B. CaSO₄
C. CaSO₄·½H₂O
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Correct Answer: Option C


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Plaster of Paris is calcium sulfate hemihydrate, CaSO₄·½H₂O. It is prepared by heating gypsum (CaSO₄·2H₂O) at 373 K. It sets with water to form gypsum.

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