ANSWER:
The correct answer is OA. 2NH3 (g) + 3Mg (s) →3H2 (g) + Mg3N2 (s)
The Equation; 2NH3 (g) + 3Mg (s) →3H2 (g) + Mg3N2 (s), is a balanced chemical equation.
FURTHER EXPLANATION;
According to the law of conservation of mass, all chemical equations should be written such that the mass of the reactants is equal to the mass of the products.
This is done by balancing the chemical equation.
Balancing of chemical equations is a try and error method that makes sure the number of atoms of each element in a chemical equation are equal on the side of the reactants and that of products.
The equation; 2NH3 (g) + 3Mg (s) →3H2 (g) + Mg3N2 is said to be balanced since the number of atoms of each element are equal on both side of the equation.
That is; 2 atoms of nitrogen, 6 atoms of hydrogen and 3 atoms of magnesium on both side of the equation.
Choice OB is incorrect because, the equation 4NH3 (g) + Mg(s)* - H2(g) + Mg3N2 (s) is not balanced as the number of nitrogen atoms and that of hydrogen atoms are not equal on both side of the equation.(4 nitrogen atoms on the reactant side and 2 nitrogen atoms on the product side, 12 hydrogen atoms on the reactant side and 2 hydrogen atoms on the product side).
Choice OC is incorrect because, the equation, NH3 (g) + Mg (s) - H2 (g) + Mg3N2 (s) is not balanced as the number of nitrogen atoms, magnesium atoms and that of hydrogen atoms are not equal on both side of the equation.(1 nitrogen atom on the reactant side and 2 nitrogen atoms on the product side, 3 hydrogen atoms on the reactant side and 2 hydrogen atoms on the product side, 1 magnesium atom on the reactant side and 3 magnesium atom on the product side).
Choice OD is incorrect because, the equation Ca (s) + 02 (9) - 2Cao (s) is not balanced as the number calcium atoms is not the same on both side of the equation, 1 calcium atom on the reactant side and 2 calcium atoms on the product side.