BaCl2 + Na2SO4 (reactants) -------------->   BaSO4 + NaCl (products)
here on reactants side Barium (Ba), Sulphur and Oxygen are already balanced with products side
But Chlorine (Cl) and Sodium(Na) aren't balanced on 2 sides. because there are 2 Cl atoms and Na are there on reactants side
In order to balance them we have to keep 2 before NaCl on products side
So, keep 2 before Nacl
Now they are balanced and hence the balanced equation is:
BaCl2 + Na2SO4 --------> BaSO4+2NaCl
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