An element with atomic number 15 belongs to which group and period? MCQ with Answer and Explanation

An element with atomic number 15 belongs to which group and period?
A. Group 3, Period 15
B. Group 5, Period 3
C. Group 15, Period 3
D. Group 15, Period 4
Answer: Option C
Solution (By JKExamLibrary)
Atomic number 15 is phosphorus. Electronic configuration: 2, 8, 5. Number of shells = 3 (Period 3). Valence electrons = 5, so it is in group 15 (10+5) for p-block elements. Group 5 is for d-block. Thus Group 15, Period 3.

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Question #1
Which of the following is an example of a strong base?
A. Mg(OH)₂
B. Ca(OH)₂ (slightly soluble)
C. NH₄OH
D. NaOH

Correct Answer: Option D


Explanation:
NaOH is a strong base, fully dissociating into Na⁺ and OH⁻ ions. NH₄OH is a weak base. Ca(OH)₂ and Mg(OH)₂ are sparingly soluble but are considered strong bases in terms of dissociation of dissolved portion, but NaOH is the strongest among common ones.

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Question #2
Radioactive isotope used in dating ancient objects is:
A. Carbon-14
B. Iodine-131
C. Cobalt-60
D. Uranium-235

Correct Answer: Option A


Explanation:
Carbon-14 dating is used to determine age of organic materials up to ~50,000 years based on its half-life of 5730 years. Cobalt-60 for cancer therapy, iodine-131 for thyroid, uranium-235 for nuclear fuel.

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Question #3
Plaster of Paris has the chemical formula:
A. CaSO₄·½H₂O
B. 2CaSO₄·H₂O
C. CaSO₄
D. CaSO₄·2H₂O

Correct Answer: Option A


Explanation:
Plaster of Paris is calcium sulfate hemihydrate, CaSO₄·½H₂O. It is obtained by heating gypsum (CaSO₄·2H₂O) at 373 K. It sets to a hard mass on mixing with water reforming gypsum. Option B (2CaSO₄·H₂O) is equivalent to CaSO₄·½H₂O but the conventional representation is CaSO₄·½H₂O. Option A is gypsum, D is anhydrous calcium sulfate. POP is used for making casts, statues, and as a building material.

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