Which pair of formulas is used to calculate pH and pOH?

• A. pH = -log[H+], pOH = -log[OH-].
• B. pH = log[H+], pOH = log[OH-].
• C. pH = -[H+], pOH = -[OH-].
• D. pH = -log[OH-], pOH = -log[H+].

Answer: (A)

pH and pOH are defined as logarithmic measures of ion concentrations, turned into a convenient, decreasing scale. The formulas are pH = -log10[H+] and pOH = -log10[OH−]. The negative sign is essential because a higher hydrogen-ion concentration means a more acidic solution, which should yield a smaller pH value, and likewise for hydroxide ions and pOH. If you omit the negative sign and just take the log, the scale would rise with acidity, which contradicts how pH is interpreted. Using a negative sign in front of the concentrations rather than inside the logarithm would not produce the standard pH/pOH relationship, and swapping the ions (using OH− for pH or H+ for pOH) would misrepresent what each measure tracks. Thus the given forms correctly capture how acidity and basicity are quantified.