Relationship Between pH and pOH for acids and bases pH vs. pOH scale: [H+] vs. [OH-]: Name ______________________________________________ Date _______________ Period _____________ Both acids and bases can measured using the pH or pOH scale. Both scales provide a measure of either the H+ concentration or the OHconcentration. Notice that each scale shows were acids and bases both are located. • When acids are measured, the pH is less than 7, but the pOH is greater than 7. • When bases are measured, the pH is greater than 7, but the pOH is less than 7. Both scales are dependent on what ion you are measuring. pH and pOH are related to one another; THEY ARE NOT INDEPENDENT OF EACH OTHER. As pH increases, pOH decreases. As pH decreases, pOH increases. By knowing what ion you are measuring on which scale, this will tell you whether or not the solution is acidic or basic. Ion Concentration Solution Type pH pOH [H+] > [OH-] Acidic pH < 7 pOH > 7 [H+] < [OH-] Basic pH > 7 pOH < 7 [H+] = [OH-] Neutral pH = 7 pOH = 7 Because these scales are related, an equation can be used to explain their correlation. pH + pOH = 14 pH = 14 − pOH pOH = 14 − pH *The bottom equations are manipulations of the top equation* The KEYS to calculations are knowing: 1. Knowing what type of solution you are working with 2. What equation to use first Example Calculations: 1. Calculate the pH and pOH of a 0.33 M H2SO4 solution. • Are you working with an acid or a base? ____________ • Are you given the concentration, or Molarity of that solution? ____________ • Next, plug the concentration or Molarity into the correct equation. Because this is an acidic solution, we have to calculate pH first. pH = − log[0.33] = ___________________ Now we can calculate the pOH, because we have calculated the pH. pOH = 14 − pH = _______________ = _______________ 2. Calculate the pH and pOH of a 0.25 M NaOH solution. • Are you working with an acid or a base? ____________ • Are you given the concentration, or Molarity of that solution? ____________ • Next, plug the concentration or Molarity into the correct equation. Because this is a basic solution, we have to calculate pOH first. pOH = − log[0.25] = ___________________ Now we can calculate the pH, because we have calculated the pOH. pH = 14 − pOH = _______________ = _______________
