Mg + 2HCl → MgCl2 + H2
1. Is the equation above balanced? How do you know?

Explanation:

According to the law of conservation of mass, mass can neither be created nor be destroyed. Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.

Thus in the reactants, there are 2 atoms of hydrogen , 2 atoms of chlorine and 1 atom of magnesium.

Thus there will be 2 atoms of hydrogen , 2 atoms of chlorine and 1 atom of magnesium in the product as well.

Thus the above equation is balanced

Learn more about law of conservation of mass

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the answer is 75.0%.