Answer
$4HCl(aq)+O_{2}(g)\ \rightarrow 2H_{2}O(l)+2Cl_{2}(g)$
Work Step by Step
Add a 2 to $H_{2}O$ to balance the oxygen, then a 4 to $HCl$ to balance the hydrogen, and finally add a 2 to $Cl_{2}$ to balance the chlorine.
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