p-Block Elements: Numerical Problems Set (7)

Question

How many grams of $\text{HCl}$ are produced when $4$ moles of $\text{H}_2$ reacts completely with chlorine? Reaction: $\text{H}_2 + \text{Cl}_2 \to 2\text{HCl}$.

Solution — Step by Step

Why This Works

Stoichiometry follows directly from the balanced equation. The coefficients give the ratio of moles, not masses. Convert masses to moles, apply the ratio, convert back to masses. That’s the universal recipe.

For p-block reactions, the classic trap is unbalanced equations. Always balance first — otherwise the ratio is wrong from the start.

Alternative Method

By mass conservation: $4$ moles H$_2$ = $8$ g; complete reaction with $4$ moles Cl$_2$ = $284$ g. Total mass on right: $8 + 284 = 292$ g, which equals the HCl mass. The arithmetic confirms the answer.

Common Mistake

Students sometimes use the ratio $1:1$ (H$_2$:HCl) instead of $1:2$, forgetting the coefficient of $2$ on HCl. The mole ratio comes straight from the balanced equation — read it carefully every time, even for “simple” reactions.