BECE Science 2022 Paper

Concept:Balancing a chemical equation requires equal numbers of each atom on both sides.Solution:The given equation is:$$\mathrm{H}_2 + \mathrm{O}_2 \rightarrow \mathrm{H}_2\mathrm{O}$$On the left side, there are $2$ hydrogen atoms and $2$ oxygen atoms.On the right side, there are $2$ hydrogen atoms and only $1$ oxygen atom.To balance the oxygen atoms, place $\frac{1}{2}$ before $\mathrm{O}_2$.The balanced equation becomes:$$\mathrm{H}_2 + \frac{1}{2}\mathrm{O}_2 \rightarrow \mathrm{H}_2\mathrm{O}$$Now, hydrogen atoms are $2$ on both sides.Oxygen atoms are $2 \times \frac{1}{2} = 1$ on the left and $1$ on the right.Thus, the number required to balance the equation is $\frac{1}{2}$.Key point: With whole numbers only, the same equation is written $2\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}$ (multiply every coefficient by $2$). The options ask for the single number placed before $\mathrm{O}_2$, which is $\frac{1}{2}$.Answer:Option A ($\frac{1}{2}$)