What does the pH scale measure in an aqueous solution?

• A. The mass percent of acid present in the solution
• B. The electrical conductivity of the solution
• C. The concentration of hydroxide ions, expressed as $-\log \left({\mathrm{OH}}_{-}\right)$
• D. The concentration of hydrogen ions, expressed as $-\log \left(H_{+}\right)$

Answer: (D)