The complete equation to following equation to make a double replacement reaction is: Na2SO4 + BaCl2 = BaSO4 + 2NaCl.
A double displacement reaction is the reaction that involves the replacement of two elements.
According to this question, barium sulfate (BaSO4) and sodium chloride are produced from the chemical equation given.
This clearly suggests that Sodium sulfate is the missing reactant, hence, the complete equation to following equation to make a double replacement reaction is:
Na2SO4 + BaCl2 = BaSO4 + 2NaCl.
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