Explain why a mixture containing equal moles of strong acid and strong base has pH 7.

Explain why a mixture containing equal moles of strong acid and strong base has pH 7.

• A. They neutralize to form water and a salt; the resulting solution is neutral under ideal conditions at 25°C.
• B. The stronger reactant determines the pH.
• C. The concentrations cancel and pH cannot be predicted.
• D. Salt hydrolysis always makes the solution acidic.

Answer: (A)

When equal moles of a strong acid and a strong base mix, they react completely to form water and a salt. The acid donates H+ ions and the base donates OH− ions, which combine to make H2O in a 1:1 ratio, consuming all the reactive species. If the resulting salt comes from a strong acid and a strong base, its constituent ions (like Na+ and Cl−) do not hydrolyze significantly in water. That means nothing in the solution shifts the balance toward extra H+ or OH−, so the solution remains neutral. At 25°C, neutral water has a pH of 7, so the mixture ends up with pH ~7. This is why strong acid–strong base neutralization yields a neutral solution under standard conditions. Salts derived from weak acids or weak bases would behave differently because those ions can hydrolyze and shift the pH, but not in this case.

When equal moles of a strong acid and a strong base mix, they react completely to form water and a salt. The acid donates H+ ions and the base donates OH− ions, which combine to make H2O in a 1:1 ratio, consuming all the reactive species. If the resulting salt comes from a strong acid and a strong base, its constituent ions (like Na+ and Cl−) do not hydrolyze significantly in water. That means nothing in the solution shifts the balance toward extra H+ or OH−, so the solution remains neutral. At 25°C, neutral water has a pH of 7, so the mixture ends up with pH ~7. This is why strong acid–strong base neutralization yields a neutral solution under standard conditions.

Salts derived from weak acids or weak bases would behave differently because those ions can hydrolyze and shift the pH, but not in this case.