Balancing Chemical Equations

Question

In the balanced equation for the reaction of aluminium with dilute sulfuric acid to produce aluminium sulfate and hydrogen gas, what is the stoichiometric coefficient of $\text{H}_2\text{SO}_4$?

RankGuru · Accepted Answer

Balancing a chemical equation requires satisfying conservation of mass (atomic balance) and charge. The unbalanced reaction is: $\text{Al} + \text{H}_2\text{SO}_4 \rightarrow \text{Al}_2(\text{SO}_4)_3 + \text{H}_2$. Aluminium sulfate contains 2 Al atoms, so 2 Al atoms are needed on the left. Each $\text{H}_2\text{SO}_4$ provides one $\text{SO}_4^{2-}$, and three are needed for $\text{Al}_2(\text{SO}_4)_3$, so 3 $\text{H}_2\text{SO}_4$ molecules are required. Balanced: $2\text{Al} + 3\text{H}_2\text{SO}_4 \rightarrow \text{Al}_2(\text{SO}_4)_3 + 3\text{H}_2$. Option 1 is insufficient; only one $\text{SO}_4^{2-}$ unit would be supplied, but $\text{Al}_2(\text{SO}_4)_3$ needs three. Option 2 provides two sulfate units, still one short of three. Option 4 is too many and would leave one $\text{SO}_4^{2-}$ unit unaccounted for, violating mass balance. This is a direct NCERT exercise on balancing equations by inspection. Plausibility check: with 3 $\text{H}_2\text{SO}_4$, we have 6 H on the left, giving $3\text{H}_2$ on the right — the hydrogen balance is also satisfied.

Balancing Chemical Equations

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