"Na2⁢SO3” represents sodium sulfite. What does the 4 mean in the formula 4 Na sub 2 SO sub 3 ?

The number 4 written in front of Na₂SO₃ is a coefficient. It tells how many units of the compound are present, so 4 Na₂SO₃ means four units of sodium sulfite. It does not mean there are four elements or four sodium atoms; each unit contains two sodium atoms.

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