Which formula represents a hydronium ion?

• A. H3O^+
• B. OH^-
• C. NH4^+
• D. HCO3^-

Answer: (A)

Verified step by step guidance

1. Understand that a hydronium ion is formed when a water molecule (H2O) gains an extra proton (H+).
2. Recognize that adding a proton (H+) to H2O results in H3O+, which carries a positive charge.
3. Identify the chemical formula for hydronium ion as \(\mathrm{H_3O^+}\), where the superscript plus indicates the positive charge.
4. Compare the given options: \(\mathrm{OH^-}\) is hydroxide ion, \(\mathrm{NH_4^+}\) is ammonium ion, and \(\mathrm{HCO_3^-}\) is bicarbonate ion, none of which represent hydronium.
5. Conclude that the correct formula for the hydronium ion is \(\mathrm{H_3O^+}\).