12th Standard Chemistry: Most Repeated Board Exam Questions
Worked out from the 105 one-mark questions of the 2020–2026 public exams. The table shows how many board questions came from each chapter; below it are the questions the board asked more than once, with the answer.
The number of electrons that have a total charge of 9650 coulombs is :
A. \(6.022 \times 10^{22}\)Correct
B. \(6.22 \times 10^{23}\)
C. \(6.022 \times 10^{-34}\)
D. \(6.022 \times 10^{24}\)
Explanation. 1 F = 96500 C is the charge of 1 mole (6.022 × 10²³) of electrons. 9650 C = 0.1 F, so number of electrons = 0.1 × 6.022 × 10²³ = 6.022 × 10²².
The formation of cyanohydrin from acetone is an example of :
A. electrophilic addition
B. nucleophilic substitution
C. nucleophilic additionCorrect
D. electrophilic substitution
Explanation. The nucleophile \(CN^-\) attacks the electrophilic carbonyl carbon of acetone and the product adds \(H^+\) on oxygen, giving acetone cyanohydrin. This is nucleophilic addition.
Explanation. FeO is found as \(Fe_{0.95}O\): some \(Fe^{2+}\) ions are missing and electrical neutrality is kept by \(Fe^{3+}\) ions, which is a metal deficiency defect. Heated ZnO shows a metal excess defect instead.
