JAMB Chemistry 2019 Paper

Concept:At s.t.p., the number of atoms present is found by multiplying the number of moles of gas by the number of atoms in each molecule.Explanation:$1$ mole of butane, $\mathrm{C_4H_{10}}$, contains $4 + 10 = 14$ atoms per molecule.So it gives $1 \times 14 = 14$ moles of atoms.$4$ moles of chlorine, $\mathrm{Cl_2}$, contain $2$ atoms per molecule.So they give $4 \times 2 = 8$ moles of atoms.$3$ moles of ozone, $\mathrm{O_3}$, contain $3$ atoms per molecule.So they give $3 \times 3 = 9$ moles of atoms.$7$ moles of argon, $\mathrm{Ar}$, contain $1$ atom per molecule because argon is monatomic.So they give $7 \times 1 = 7$ moles of atoms.Since $7 < 8 < 9 < 14$, argon contains the least number of atoms.Answer:D. $7$ moles of argon.