If an aqueous solution turns blue litmus red, which relationship exists between the hydronium ion and hydroxide ion?

If an aqueous solution turns blue litmus red, which relationship exists between the hydronium ion and hydroxide ion?

• A. [H3O+] < [OH-]
• B. neither ion is present
• C. [H3O+] = [OH-]
• D. [H3O+] > [OH-]

Answer: (D)

The main idea is that acidity of an aqueous solution is determined by the balance between hydronium and hydroxide ions. When blue litmus turns red, the solution is acidic, meaning there are more hydronium ions than hydroxide ions. In water, autoionization sets Kw = [H3O+] [OH−] ≈ 1.0 × 10−14 at room temperature. If [H3O+] is greater than [OH−], their product still equals Kw, so the hydroxide concentration must be smaller to keep the product constant. This situation—hydronium dominating over hydroxide—defines an acidic solution, so the relationship is [H3O+] > [OH−]. If [H3O+] were smaller, the solution would be basic; if they were equal, the solution would be neutral.

The main idea is that acidity of an aqueous solution is determined by the balance between hydronium and hydroxide ions. When blue litmus turns red, the solution is acidic, meaning there are more hydronium ions than hydroxide ions.

In water, autoionization sets Kw = [H3O+] [OH−] ≈ 1.0 × 10−14 at room temperature. If [H3O+] is greater than [OH−], their product still equals Kw, so the hydroxide concentration must be smaller to keep the product constant. This situation—hydronium dominating over hydroxide—defines an acidic solution, so the relationship is [H3O+] > [OH−]. If [H3O+] were smaller, the solution would be basic; if they were equal, the solution would be neutral.