JEE Main · 2026 · 08 Apr 2026 · Shift 2 · Some Basic Concepts of Chemistry

The number of atoms equals (mass / molar mass) $\times$ (atoms per formula unit) $\times N_A$. For $1.8\mathrm{mg}$ water, $\frac{1.8\times 10^{-3}}{18}=10^{-4}$ mol, and water has three atoms, giving $3\times 10^{-4}{N}_A$. For $9.8\mathrm{mg}$ sulphuric acid, $\frac{9.8\times 10^{-3}}{98}=10^{-4}$ mol, and ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ has seven atoms, giving $7\times 10^{-4}{N}_A$. For $1.8\mathrm{mg}$ carbon, $\frac{1.8\times 10^{-3}}{12}=1.5\times 10^{-4}$ mol of atoms, i.e. $1.5\times 10^{-4}{N}_A$. For $5.85\mathrm{mg}$ sodium chloride, $\frac{5.85\times 10^{-3}}{58.5}=10^{-4}$ mol, and each formula unit supplies two atoms, giving $2\times 10^{-4}{N}_A$. Hence water matches $3\times 10^{-4}{N}_A$, sulphuric acid $7\times 10^{-4}{N}_A$, carbon $1.5\times 10^{-4}{N}_A$ and sodium chloride $2\times 10^{-4}{N}_A$.