Taking into account the reaction stoichiometry, 1.25 grams of H₂ (option A) are formed if you start with 2.5 moles of Magnesium and an excess of Hydrochloric Acid.
<h3>Reaction stoichiometry</h3>
In first place, the balanced reaction is:
Mg + 2 HCl → MgCl₂ + H₂
By reaction stoichiometry (that is, the relationship between the amount of reagents and products in a chemical reaction), the following amounts of moles of each compound participate in the reaction:
- Mg: 1 mole
- HCl: 2 moles
- MgCl₂: 1 mole
- H₂: 1 mole
<h3>Mass of hydrogen formed</h3>
The following rule of three can be applied: if by reaction stoichiometry 2 moles of Mg form 1 mole of H₂, 2.5 moles of Mg form how many moles of H₂?

<u><em>moles of H₂= 1.25 moles</em></u>
Finally, 1.25 grams of H₂ (option A) are formed if you start with 2.5 moles of Magnesium and an excess of Hydrochloric Acid.
Learn more about the reaction stoichiometry:
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