# Silicon tetrachloride (SiCl_4) can be prepared by heating Si in chlorine gas: Si (s) + 2Cl_2(g) -> SiCl_4(l). In one reaction, 0.507 mole of SiCl_4 is produced. How many moles of molecular chlorine were used in the reaction?

Stoichiometry demands at least $1.14$ $m o l$ of $C {l}_{2}$ gas.
The stoichiometric equation, which you have correctly written, clearly and explicitly shows that $1$ $m o l$ of silicon (approx. $28 \cdot g$) reacts with $2 \cdot \text{moles}$ of chlorine gas (approx. $71 \cdot g$) to give $1$ $m o l$ of $S i C {l}_{4}$.