Why is aqueous solution of CH3COONH4 neutral?
If you mean neutral in charge, then the answer is because an aqueous solution contains the same number of positive charges (on the cations) and negative charges (on the anions). If you mean neutral on the pH scale, then this is only approximately true. Dilute solutions of Ammonium acetate typically have a pH of 6.7 to 7.3 (nearly neutral, but not exactly 7.0). Generally speaking salts formed from the neutralization of acids and bases of about the same strength (strong with strong or weak with weak) will give solutions near neutral. Acetic acid (ethanoic acid) is a weal organic acid. Ammonium hydroxide is a weak base.
