If 1 mole of each of the following substances were dissolved in 1 liter of water, which solution would contain the highest concentration of H3O+ ions?

If 1 mole of each of the following substances were dissolved in 1 liter of water, which solution would contain the highest concentration of H3O+ ions?

• A. CH3COOH
• B. Ba(OH)2
• C. KBr
• D. NaCl

Answer: (A)

Concentrations of hydronium ions depend on whether the dissolved species act as acids, bases, or neutral salts in water. A strong acid would fully donate H3O+, a strong base would generate OH− and push pH up (reducing [H3O+]), and neutral salts from strong acid–base pairs don’t shift the pH much, leaving [H3O+] near that of pure water. A weak acid, however, partially dissociates and does contribute H3O+ to the solution, though not as completely as a strong acid. Among the options, only the weak acid provides a substantial production of H3O+. For acetic acid, with Ka about 1.8×10−5 and a 1 M solution, the equilibrium [H3O+] can be estimated using [H3O+] ≈ sqrt(Ka × C) ≈ sqrt(1.8×10−5 × 1) ≈ 4×10−3 M. This is far larger than the ~1×10−7 M from neutral water or from a solution made with a strong base like Ba(OH)2, which makes the solution basic and lowers [H3O+]. The salts KBr and NaCl produce solutions that are essentially neutral, giving [H3O+] around 1×10−7 M. Therefore the acetic acid solution has the highest concentration of hydronium ions.

Concentrations of hydronium ions depend on whether the dissolved species act as acids, bases, or neutral salts in water. A strong acid would fully donate H3O+, a strong base would generate OH− and push pH up (reducing [H3O+]), and neutral salts from strong acid–base pairs don’t shift the pH much, leaving [H3O+] near that of pure water. A weak acid, however, partially dissociates and does contribute H3O+ to the solution, though not as completely as a strong acid. Among the options, only the weak acid provides a substantial production of H3O+. For acetic acid, with Ka about 1.8×10−5 and a 1 M solution, the equilibrium [H3O+] can be estimated using [H3O+] ≈ sqrt(Ka × C) ≈ sqrt(1.8×10−5 × 1) ≈ 4×10−3 M. This is far larger than the ~1×10−7 M from neutral water or from a solution made with a strong base like Ba(OH)2, which makes the solution basic and lowers [H3O+]. The salts KBr and NaCl produce solutions that are essentially neutral, giving [H3O+] around 1×10−7 M. Therefore the acetic acid solution has the highest concentration of hydronium ions.