WAEC Chemistry 2011 Paper

Concept:The amount of a substance in moles is found by dividing the number of particles by Avogadro's constant.The atomic mass of magnesium, $24$, is not needed because this question asks for moles, not mass in grams.Explanation:Number of particles: $N = 1.20 \times 10^{24}$Avogadro's constant: $N_A = 6.02 \times 10^{23}$ particles/molUsing the relation:$$\text{Amount in moles} = \frac{N}{N_A}$$Substituting the values:$$\text{Amount in moles} = \frac{1.20 \times 10^{24}}{6.02 \times 10^{23}}$$Simplifying the fraction:$$\frac{1.20}{6.02} \times 10^{24-23} = 0.199 \times 10^{1} = 1.99$$Rounding gives approximately $2.0$ moles.Therefore, $1.20 \times 10^{24}$ particles of magnesium correspond to $2.0$ moles of magnesium atoms.Answer:B. 2.0 moles