NumericalModerate2 marks
1(e)(i)
Calculate:
The weight of 1022 atoms of carbon. [C = 12, Avogadro’s No. = 6 x 1023]
Topic: Mole concept, gas volumes and stoichiometry
Previous-year question practice database
Answer
Exam answer
Given: 10²² atoms of carbon. [C = 12, Avogadro's number = 6 × 10²³]
Moles = number of atoms ÷ Avogadro's number = 10²² ÷ (6 × 10²³) = 1 ÷ 60 = 0.0167 mol
Mass = moles × atomic mass = (1 ÷ 60) × 12 = 12 ÷ 60 = 0.2 g
Weight of 10²² atoms of carbon = 0.2 g
Explanation
Keep the powers of ten as a fraction rather than converting to decimals early: 10²² ÷ (6 × 10²³) is 1 ÷ 60, which stays exact all the way to 12 ÷ 60 = 0.2. Going to 0.0167 first and then multiplying invites a rounding error.
Carbon is a solid element, so the particle is a single ATOM and the mass to use is 12 — there is no C₂. Sanity check: 10²² is much less than Avogadro's number, so the answer must be well under 12 g.
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