Question

how does the law of conservation of mass apply to this reaction: mg + hcl → h₂ + mgcl₂?
the law of conservation of mass has already been applied. there is an equal number of each
element on both sides of the equation.
the equation needs to be balanced. there are fewer hydrogen atoms in the equation than
magnesium or chlorine.
only the hydrogen needs to be balanced. there are equal numbers of magnesium and chlorine.
hydrogen and chlorine need to be balanced. there is an equal amount of magnesium on each
side

Explanation:

Step1: Count atoms on both sides

Reactants: \(Mg\) (1 atom), \(HCl\) (1 \(H\) and 1 \(Cl\) atom). Products: \(H_2\) (2 \(H\) atoms), \(MgCl_2\) (1 \(Mg\) and 2 \(Cl\) atoms).

Step2: Analyze imbalance

For \(H\): 1 (reactant) vs 2 (product). For \(Cl\): 1 (reactant) vs 2 (product). \(Mg\) is balanced (1 on each side). So, equation is unbalanced. Hydrogen and chlorine need balancing. First option is wrong as equation is unbalanced. Third option is wrong as both \(H\) and \(Cl\) need balancing. Fourth option is wrong as \(Cl\) is also unbalanced. Second option is correct as equation is unbalanced with fewer \(H\) (and \(Cl\)) on reactant side compared to product side (for \(Cl\)).

Answer:

The equation needs to be balanced. There are fewer hydrogen atoms in the equation than magnesium or chlorine.