Q. \[ \mathrm{Mg} + \mathrm{HCl} \]

Step 1: Identify reactants and likely products. The reactants are magnesium and hydrochloric acid, written as \( \mathrm{Mg} \) and \( \mathrm{HCl} \). When a metal like magnesium reacts with hydrochloric acid, the typical products are the corresponding metal chloride and hydrogen gas. Those products are \( \mathrm{MgCl}_2 \) and \( \mathrm{H}_2 \).

Step 2: Write the unbalanced chemical equation using chemical formulas. The unbalanced equation is

\[ \mathrm{Mg} + \mathrm{HCl} = \mathrm{MgCl}_2 + \mathrm{H}_2 \]

Step 3: Count atoms of each element on both sides of the unbalanced equation.

Left side atom counts per molecule: magnesium 1, hydrogen 1, chlorine 1. In math form:

\( \text{Left: } \mathrm{Mg}: 1,\; \mathrm{H}: 1,\; \mathrm{Cl}: 1 \)

Right side atom counts per molecule: magnesium in \( \mathrm{MgCl}_2 \) is 1, chlorine in \( \mathrm{MgCl}_2 \) is 2, hydrogen in \( \mathrm{H}_2 \) is 2. In math form:

\( \text{Right: } \mathrm{Mg}: 1,\; \mathrm{H}: 2,\; \mathrm{Cl}: 2 \)

Step 4: Determine coefficients to balance each element. Magnesium is already balanced with 1 on each side. Chlorine and hydrogen are not balanced. Each molecule of \( \mathrm{MgCl}_2 \) contains 2 chlorine atoms, so place coefficient 2 before \( \mathrm{HCl} \) to provide 2 chlorine atoms on the left. That also changes the hydrogen count on the left to 2, which matches the hydrogen count on the right.

Step 5: Write the equation with the chosen coefficients and verify all atom counts match. With coefficient 2 before \( \mathrm{HCl} \) the equation becomes

\[ \mathrm{Mg} + 2\,\mathrm{HCl} = \mathrm{MgCl}_2 + \mathrm{H}_2 \]

Verify counts after balancing: Left side: \( \mathrm{Mg}: 1,\; \mathrm{Cl}: 2,\; \mathrm{H}: 2 \). Right side: \( \mathrm{Mg}: 1,\; \mathrm{Cl}: 2,\; \mathrm{H}: 2 \). All elements are balanced.

Final balanced equation:

\[ \mathrm{Mg} + 2\,\mathrm{HCl} = \mathrm{MgCl}_2 + \mathrm{H}_2 \]