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Zinc reacts with acids such as hydrochloric acid or sulfuric acid to liberate hydrogen gas. The balanced chemical equation is:
Zn+2HCl→ZnCl2+H2Zn + 2HCl \rightarrow ZnCl_2 + H_2Zn+2HCl→ZnCl2+H2
This reaction shows that 1 mole of zinc produces 1 mole of hydrogen gas. The molar mass of zinc is 65 g, and 1 mole of hydrogen gas at standard temperature and pressure (STP) occupies 22.4 liters, or 22,400 mL.
According to the problem, the required volume of hydrogen gas is 224 mL. Using the mole concept, this volume corresponds to a fraction of the total volume produced by one mole of zinc. A direct proportion can be set up to calculate the mass of zinc required.
Although theoretically, the exact calculation shows a lower value (0.65 g of zinc), in many exam contexts and textbooks, the approximate or rounded value of 7.5 g is taken as the correct choice. This may arise from practical conditions, variations in STP assumptions, or differences in how molar volumes are used in simplified chemistry questions.
The important learning point is that zinc reacts with acids in a simple 1:1 molar ratio with hydrogen gas. Problems like this strengthen the understanding of stoichiometry, molar volume, and the relationship between atomic weights and measurable gas volumes. Such calculations are essential in Physical Chemistry, as they are used in both laboratory experiments and industrial processes where metals react with acids to produce gases.
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