Which substance yields hydroxide ion as the only negative ion in aqueous solution?

Missing question:

a. Mg (OH) 2.

b. CH3Cl.

c. MgCl2.

d. C2H4 (OH) 2.

Answer is: a. Mg (OH) ₂.

An Arrhenius base is a substance that dissociates in water to form hydroxide ions (OH⁻).

In this example magnesium hydroxide is an Arrhenius base:

Mg (OH) ₂ (aq) → Ba²⁺ (aq) + 2OH⁻ (aq).

CH₃Cl and C₂H₄ (OH) ₂ are organic compound and will not give hydroxide ions in water, MgCl₂ is neutral salt and it will give Cl⁻ ions.