how do you balance this equation?
2SO2(g)+ O2> SO3(g)?

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Based on what you have written here, all that needs to be done is to add a 2 to the front of your product.

The result is...

2 SO2 + O2 --> 2 SO3

If we count the atoms now we find that there are

2 sulfur and
6 oxygen

On both sides.