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MCQ practice · SSC Chemistry Board questions · Chapter 6: Concept of Mole and Chemical Counting

Sylhet Board 2022

What is the mass of 4 Mg atoms?

Think it through first, then pick an option.

  1. a 7.96 × 10⁻²³ gm
  2. b 1.59 × 10⁻²² gm
  3. c 48 gm
  4. d 96 gm
Answer and explanation

Correct answer: b 1.59 × 10⁻²² gm

One Mg = 24/6.023 × 10²³ = 3.98 × 10⁻²³ g; four = 1.59 × 10⁻²² g.

Why the other options are wrong

  • a 7.96 × 10⁻²³ gm: 7.96 × 10⁻²³ g is two Mg atoms; four weigh 4 × 24/6.023 × 10²³ = 1.59 × 10⁻²² g.
  • c 48 gm: 48 g is the mass of 2 moles of Mg, not of four atoms.
  • d 96 gm: 96 g is 4 moles of Mg; four atoms weigh 1.59 × 10⁻²² g.