Some Basic Concepts of Chemistry Part 3

Concept:Use stoichiometric coefficients from the balanced chemical equation to relate moles of reactants and products. At STP (standard temperature and pressure), 1 mole of any gas occupies 22.4 L.Chemical Equation / Formula:$2\mathrm{Al(s)} + 6\mathrm{HCl(aq)} \rightarrow 2\mathrm{Al}^{3+}\mathrm{(aq)} + 6\mathrm{Cl}^{-}\mathrm{(aq)} + 3\mathrm{H}_2\mathrm{(g)}$Explanation:From the balanced equation, 6 moles of HCl produce 3 moles of $\mathrm{H}_2$.Thus, moles of $\mathrm{H}_2$ per mole of HCl = $\frac{3}{6} = \frac{1}{2}$.At STP, 1 mole $\mathrm{H}_2$ = 22.4 L, so $\frac{1}{2}$ mole $\mathrm{H}_2$ = 11.2 L.Hence, for every mole of HCl consumed, 11.2 L of $\mathrm{H}_2$ is produced at STP.Other options are incorrect:• Option A: HCl is aqueous, not gaseous, so direct volume relation fails.• Option C: Volume of gas depends on temperature and pressure; 33.6 L is only valid at STP for 1 mole Al, not "regardless".• Option D: 1 mole Al gives 1.5 moles $\mathrm{H}_2$ = 33.6 L at STP, not 67.2 L.Shortcut:Ratio of $\mathrm{H}_2$ to HCl = 3:6 = 1:2. So volume of $\mathrm{H}_2$ at STP = $\frac{1}{2} \times 22.4$ L per mole HCl = 11.2 L.Answer:Option B: 11.2 L $\mathrm{H}_2$(g) at STP is produced for every mole of HCl consumed.