Assuming solid Mg as your starting material, the only thing you could really eliminate as a spectator would be the butyric acid chain, that is, CH3CH2CH2COO-, from both sides of the equation.
Since all the other atoms have changed in some way during the reaction, your answer looks good - excellent work!
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Since all the other atoms have changed in some way during the reaction, your answer looks good - excellent work!
♠