If a solution has pH 3, how does its acidity compare to a solution with pH 5?

• A. 10x more acidic
• B. Equally acidic
• C. 100x less acidic
• D. 100x more acidic

Answer: (D)

acidity is reflected by the hydrogen ion concentration, and the pH scale is a negative logarithm: each unit change in pH corresponds to a tenfold change in [H+]. So going from pH 3 to pH 5 lowers the hydrogen ion concentration by a factor of 10^2, meaning the solution at pH 3 has 100 times more H+ than the one at pH 5. That makes it 100x more acidic. The pH 5 solution is therefore 100x less acidic than the pH 3 solution. A difference of one pH unit would be 10x, not 100x, and equal acidity would require the same pH.