A solution with pH 3 is acidic; is it stronger or weaker acid than a solution with pH 5?

A solution with pH 3 is acidic; is it stronger or weaker acid than a solution with pH 5?

• A. Acidic; stronger than pH 5
• B. Acidic; weaker than pH 5
• C. Basic; stronger than pH 5
• D. Basic; weaker than pH 5

Answer: (A)

Understanding pH and hydrogen ion concentration is the key. pH is the negative logarithm of [H+], so lower pH means more hydrogen ions in solution. A solution with pH 3 has [H+] = 1×10^-3 M, while a solution with pH 5 has [H+] = 1×10^-5 M. That’s a 100-fold difference in hydrogen ion concentration, making the pH 3 solution much more acidic. Since both are below 7, they’re acidic, but the pH 3 solution is the stronger acid because it has the higher [H+].

Understanding pH and hydrogen ion concentration is the key. pH is the negative logarithm of [H+], so lower pH means more hydrogen ions in solution. A solution with pH 3 has [H+] = 1×10^-3 M, while a solution with pH 5 has [H+] = 1×10^-5 M. That’s a 100-fold difference in hydrogen ion concentration, making the pH 3 solution much more acidic. Since both are below 7, they’re acidic, but the pH 3 solution is the stronger acid because it has the higher [H+].